linux/arch/mips/include/asm/processor.h

403 lines
11 KiB

MIPS: Use per-mm page to execute branch delay slot instructions In some cases the kernel needs to execute an instruction from the delay slot of an emulated branch instruction. These cases include: - Emulated floating point branch instructions (bc1[ft]l?) for systems which don't include an FPU, or upon which the kernel is run with the "nofpu" parameter. - MIPSr6 systems running binaries targeting older revisions of the architecture, which may include branch instructions whose encodings are no longer valid in MIPSr6. Executing instructions from such delay slots is done by writing the instruction to memory followed by a trap, as part of an "emuframe", and executing it. This avoids the requirement of an emulator for the entire MIPS instruction set. Prior to this patch such emuframes are written to the user stack and executed from there. This patch moves FP branch delay emuframes off of the user stack and into a per-mm page. Allocating a page per-mm leaves userland with access to only what it had access to previously, and compared to other solutions is relatively simple. When a thread requires a delay slot emulation, it is allocated a frame. A thread may only have one frame allocated at any one time, since it may only ever be executing one instruction at any one time. In order to ensure that we can free up allocated frame later, its index is recorded in struct thread_struct. In the typical case, after executing the delay slot instruction we'll execute a break instruction with the BRK_MEMU code. This traps back to the kernel & leads to a call to do_dsemulret which frees the allocated frame & moves the user PC back to the instruction that would have executed following the emulated branch. In some cases the delay slot instruction may be invalid, such as a branch, or may trigger an exception. In these cases the BRK_MEMU break instruction will not be hit. In order to ensure that frames are freed this patch introduces dsemul_thread_cleanup() and calls it to free any allocated frame upon thread exit. If the instruction generated an exception & leads to a signal being delivered to the thread, or indeed if a signal simply happens to be delivered to the thread whilst it is executing from the struct emuframe, then we need to take care to exit the frame appropriately. This is done by either rolling back the user PC to the branch or advancing it to the continuation PC prior to signal delivery, using dsemul_thread_rollback(). If this were not done then a sigreturn would return to the struct emuframe, and if that frame had meanwhile been used in response to an emulated branch instruction within the signal handler then we would execute the wrong user code. Whilst a user could theoretically place something like a compact branch to self in a delay slot and cause their thread to become stuck in an infinite loop with the frame never being deallocated, this would: - Only affect the users single process. - Be architecturally invalid since there would be a branch in the delay slot, which is forbidden. - Be extremely unlikely to happen by mistake, and provide a program with no more ability to harm the system than a simple infinite loop would. If a thread requires a delay slot emulation & no frame is available to it (ie. the process has enough other threads that all frames are currently in use) then the thread joins a waitqueue. It will sleep until a frame is freed by another thread in the process. Since we now know whether a thread has an allocated frame due to our tracking of its index, the cookie field of struct emuframe is removed as we can be more certain whether we have a valid frame. Since a thread may only ever have a single frame at any given time, the epc field of struct emuframe is also removed & the PC to continue from is instead stored in struct thread_struct. Together these changes simplify & shrink struct emuframe somewhat, allowing twice as many frames to fit into the page allocated for them. The primary benefit of this patch is that we are now free to mark the user stack non-executable where that is possible. Signed-off-by: Paul Burton <paul.burton@imgtec.com> Cc: Leonid Yegoshin <leonid.yegoshin@imgtec.com> Cc: Maciej Rozycki <maciej.rozycki@imgtec.com> Cc: Faraz Shahbazker <faraz.shahbazker@imgtec.com> Cc: Raghu Gandham <raghu.gandham@imgtec.com> Cc: Matthew Fortune <matthew.fortune@imgtec.com> Cc: linux-mips@linux-mips.org Patchwork: https://patchwork.linux-mips.org/patch/13764/ Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
2016-07-08 11:06:19 +01:00
MIPS: VDSO: Always map near top of user memory When using the legacy mmap layout, for example triggered using ulimit -s unlimited, get_unmapped_area() fills memory from bottom to top starting from a fairly low address near TASK_UNMAPPED_BASE. This placement is suboptimal if the user application wishes to allocate large amounts of heap memory using the brk syscall. With the VDSO being located low in the user's virtual address space, the amount of space available for access using brk is limited much more than it was prior to the introduction of the VDSO. For example: # ulimit -s unlimited; cat /proc/self/maps 00400000-004ec000 r-xp 00000000 08:00 71436 /usr/bin/coreutils 004fc000-004fd000 rwxp 000ec000 08:00 71436 /usr/bin/coreutils 004fd000-0050f000 rwxp 00000000 00:00 0 00cc3000-00ce4000 rwxp 00000000 00:00 0 [heap] 2ab96000-2ab98000 r--p 00000000 00:00 0 [vvar] 2ab98000-2ab99000 r-xp 00000000 00:00 0 [vdso] 2ab99000-2ab9d000 rwxp 00000000 00:00 0 ... Resolve this by adjusting STACK_TOP to reserve space for the VDSO & providing an address hint to get_unmapped_area() causing it to use this space even when using the legacy mmap layout. We reserve enough space for the VDSO, plus 1MB or 256MB for 32 bit & 64 bit systems respectively within which we randomize the VDSO base address. Previously this randomization was taken care of by the mmap base address randomization performed by arch_mmap_rnd(). The 1MB & 256MB sizes are somewhat arbitrary but chosen such that we have some randomization without taking up too much of the user's virtual address space, which is often in short supply for 32 bit systems. With this the VDSO is always mapped at a high address, leaving lots of space for statically linked programs to make use of brk: # ulimit -s unlimited; cat /proc/self/maps 00400000-004ec000 r-xp 00000000 08:00 71436 /usr/bin/coreutils 004fc000-004fd000 rwxp 000ec000 08:00 71436 /usr/bin/coreutils 004fd000-0050f000 rwxp 00000000 00:00 0 00c28000-00c49000 rwxp 00000000 00:00 0 [heap] ... 7f67c000-7f69d000 rwxp 00000000 00:00 0 [stack] 7f7fc000-7f7fd000 rwxp 00000000 00:00 0 7fcf1000-7fcf3000 r--p 00000000 00:00 0 [vvar] 7fcf3000-7fcf4000 r-xp 00000000 00:00 0 [vdso] Signed-off-by: Paul Burton <paul.burton@mips.com> Reported-by: Huacai Chen <chenhc@lemote.com> Fixes: ebb5e78cc634 ("MIPS: Initial implementation of a VDSO") Cc: Huacai Chen <chenhc@lemote.com> Cc: linux-mips@linux-mips.org Cc: stable@vger.kernel.org # v4.4+
2018-09-25 15:51:26 -07:00
MIPS: Use per-mm page to execute branch delay slot instructions In some cases the kernel needs to execute an instruction from the delay slot of an emulated branch instruction. These cases include: - Emulated floating point branch instructions (bc1[ft]l?) for systems which don't include an FPU, or upon which the kernel is run with the "nofpu" parameter. - MIPSr6 systems running binaries targeting older revisions of the architecture, which may include branch instructions whose encodings are no longer valid in MIPSr6. Executing instructions from such delay slots is done by writing the instruction to memory followed by a trap, as part of an "emuframe", and executing it. This avoids the requirement of an emulator for the entire MIPS instruction set. Prior to this patch such emuframes are written to the user stack and executed from there. This patch moves FP branch delay emuframes off of the user stack and into a per-mm page. Allocating a page per-mm leaves userland with access to only what it had access to previously, and compared to other solutions is relatively simple. When a thread requires a delay slot emulation, it is allocated a frame. A thread may only have one frame allocated at any one time, since it may only ever be executing one instruction at any one time. In order to ensure that we can free up allocated frame later, its index is recorded in struct thread_struct. In the typical case, after executing the delay slot instruction we'll execute a break instruction with the BRK_MEMU code. This traps back to the kernel & leads to a call to do_dsemulret which frees the allocated frame & moves the user PC back to the instruction that would have executed following the emulated branch. In some cases the delay slot instruction may be invalid, such as a branch, or may trigger an exception. In these cases the BRK_MEMU break instruction will not be hit. In order to ensure that frames are freed this patch introduces dsemul_thread_cleanup() and calls it to free any allocated frame upon thread exit. If the instruction generated an exception & leads to a signal being delivered to the thread, or indeed if a signal simply happens to be delivered to the thread whilst it is executing from the struct emuframe, then we need to take care to exit the frame appropriately. This is done by either rolling back the user PC to the branch or advancing it to the continuation PC prior to signal delivery, using dsemul_thread_rollback(). If this were not done then a sigreturn would return to the struct emuframe, and if that frame had meanwhile been used in response to an emulated branch instruction within the signal handler then we would execute the wrong user code. Whilst a user could theoretically place something like a compact branch to self in a delay slot and cause their thread to become stuck in an infinite loop with the frame never being deallocated, this would: - Only affect the users single process. - Be architecturally invalid since there would be a branch in the delay slot, which is forbidden. - Be extremely unlikely to happen by mistake, and provide a program with no more ability to harm the system than a simple infinite loop would. If a thread requires a delay slot emulation & no frame is available to it (ie. the process has enough other threads that all frames are currently in use) then the thread joins a waitqueue. It will sleep until a frame is freed by another thread in the process. Since we now know whether a thread has an allocated frame due to our tracking of its index, the cookie field of struct emuframe is removed as we can be more certain whether we have a valid frame. Since a thread may only ever have a single frame at any given time, the epc field of struct emuframe is also removed & the PC to continue from is instead stored in struct thread_struct. Together these changes simplify & shrink struct emuframe somewhat, allowing twice as many frames to fit into the page allocated for them. The primary benefit of this patch is that we are now free to mark the user stack non-executable where that is possible. Signed-off-by: Paul Burton <paul.burton@imgtec.com> Cc: Leonid Yegoshin <leonid.yegoshin@imgtec.com> Cc: Maciej Rozycki <maciej.rozycki@imgtec.com> Cc: Faraz Shahbazker <faraz.shahbazker@imgtec.com> Cc: Raghu Gandham <raghu.gandham@imgtec.com> Cc: Matthew Fortune <matthew.fortune@imgtec.com> Cc: linux-mips@linux-mips.org Patchwork: https://patchwork.linux-mips.org/patch/13764/ Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
2016-07-08 11:06:19 +01:00
MIPS: VDSO: Always map near top of user memory When using the legacy mmap layout, for example triggered using ulimit -s unlimited, get_unmapped_area() fills memory from bottom to top starting from a fairly low address near TASK_UNMAPPED_BASE. This placement is suboptimal if the user application wishes to allocate large amounts of heap memory using the brk syscall. With the VDSO being located low in the user's virtual address space, the amount of space available for access using brk is limited much more than it was prior to the introduction of the VDSO. For example: # ulimit -s unlimited; cat /proc/self/maps 00400000-004ec000 r-xp 00000000 08:00 71436 /usr/bin/coreutils 004fc000-004fd000 rwxp 000ec000 08:00 71436 /usr/bin/coreutils 004fd000-0050f000 rwxp 00000000 00:00 0 00cc3000-00ce4000 rwxp 00000000 00:00 0 [heap] 2ab96000-2ab98000 r--p 00000000 00:00 0 [vvar] 2ab98000-2ab99000 r-xp 00000000 00:00 0 [vdso] 2ab99000-2ab9d000 rwxp 00000000 00:00 0 ... Resolve this by adjusting STACK_TOP to reserve space for the VDSO & providing an address hint to get_unmapped_area() causing it to use this space even when using the legacy mmap layout. We reserve enough space for the VDSO, plus 1MB or 256MB for 32 bit & 64 bit systems respectively within which we randomize the VDSO base address. Previously this randomization was taken care of by the mmap base address randomization performed by arch_mmap_rnd(). The 1MB & 256MB sizes are somewhat arbitrary but chosen such that we have some randomization without taking up too much of the user's virtual address space, which is often in short supply for 32 bit systems. With this the VDSO is always mapped at a high address, leaving lots of space for statically linked programs to make use of brk: # ulimit -s unlimited; cat /proc/self/maps 00400000-004ec000 r-xp 00000000 08:00 71436 /usr/bin/coreutils 004fc000-004fd000 rwxp 000ec000 08:00 71436 /usr/bin/coreutils 004fd000-0050f000 rwxp 00000000 00:00 0 00c28000-00c49000 rwxp 00000000 00:00 0 [heap] ... 7f67c000-7f69d000 rwxp 00000000 00:00 0 [stack] 7f7fc000-7f7fd000 rwxp 00000000 00:00 0 7fcf1000-7fcf3000 r--p 00000000 00:00 0 [vvar] 7fcf3000-7fcf4000 r-xp 00000000 00:00 0 [vdso] Signed-off-by: Paul Burton <paul.burton@mips.com> Reported-by: Huacai Chen <chenhc@lemote.com> Fixes: ebb5e78cc634 ("MIPS: Initial implementation of a VDSO") Cc: Huacai Chen <chenhc@lemote.com> Cc: linux-mips@linux-mips.org Cc: stable@vger.kernel.org # v4.4+
2018-09-25 15:51:26 -07:00
MIPS: Use per-mm page to execute branch delay slot instructions In some cases the kernel needs to execute an instruction from the delay slot of an emulated branch instruction. These cases include: - Emulated floating point branch instructions (bc1[ft]l?) for systems which don't include an FPU, or upon which the kernel is run with the "nofpu" parameter. - MIPSr6 systems running binaries targeting older revisions of the architecture, which may include branch instructions whose encodings are no longer valid in MIPSr6. Executing instructions from such delay slots is done by writing the instruction to memory followed by a trap, as part of an "emuframe", and executing it. This avoids the requirement of an emulator for the entire MIPS instruction set. Prior to this patch such emuframes are written to the user stack and executed from there. This patch moves FP branch delay emuframes off of the user stack and into a per-mm page. Allocating a page per-mm leaves userland with access to only what it had access to previously, and compared to other solutions is relatively simple. When a thread requires a delay slot emulation, it is allocated a frame. A thread may only have one frame allocated at any one time, since it may only ever be executing one instruction at any one time. In order to ensure that we can free up allocated frame later, its index is recorded in struct thread_struct. In the typical case, after executing the delay slot instruction we'll execute a break instruction with the BRK_MEMU code. This traps back to the kernel & leads to a call to do_dsemulret which frees the allocated frame & moves the user PC back to the instruction that would have executed following the emulated branch. In some cases the delay slot instruction may be invalid, such as a branch, or may trigger an exception. In these cases the BRK_MEMU break instruction will not be hit. In order to ensure that frames are freed this patch introduces dsemul_thread_cleanup() and calls it to free any allocated frame upon thread exit. If the instruction generated an exception & leads to a signal being delivered to the thread, or indeed if a signal simply happens to be delivered to the thread whilst it is executing from the struct emuframe, then we need to take care to exit the frame appropriately. This is done by either rolling back the user PC to the branch or advancing it to the continuation PC prior to signal delivery, using dsemul_thread_rollback(). If this were not done then a sigreturn would return to the struct emuframe, and if that frame had meanwhile been used in response to an emulated branch instruction within the signal handler then we would execute the wrong user code. Whilst a user could theoretically place something like a compact branch to self in a delay slot and cause their thread to become stuck in an infinite loop with the frame never being deallocated, this would: - Only affect the users single process. - Be architecturally invalid since there would be a branch in the delay slot, which is forbidden. - Be extremely unlikely to happen by mistake, and provide a program with no more ability to harm the system than a simple infinite loop would. If a thread requires a delay slot emulation & no frame is available to it (ie. the process has enough other threads that all frames are currently in use) then the thread joins a waitqueue. It will sleep until a frame is freed by another thread in the process. Since we now know whether a thread has an allocated frame due to our tracking of its index, the cookie field of struct emuframe is removed as we can be more certain whether we have a valid frame. Since a thread may only ever have a single frame at any given time, the epc field of struct emuframe is also removed & the PC to continue from is instead stored in struct thread_struct. Together these changes simplify & shrink struct emuframe somewhat, allowing twice as many frames to fit into the page allocated for them. The primary benefit of this patch is that we are now free to mark the user stack non-executable where that is possible. Signed-off-by: Paul Burton <paul.burton@imgtec.com> Cc: Leonid Yegoshin <leonid.yegoshin@imgtec.com> Cc: Maciej Rozycki <maciej.rozycki@imgtec.com> Cc: Faraz Shahbazker <faraz.shahbazker@imgtec.com> Cc: Raghu Gandham <raghu.gandham@imgtec.com> Cc: Matthew Fortune <matthew.fortune@imgtec.com> Cc: linux-mips@linux-mips.org Patchwork: https://patchwork.linux-mips.org/patch/13764/ Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
2016-07-08 11:06:19 +01:00
2015-01-08 12:17:37 +00:00