Revert "riscv: Define TASK_SIZE_MAX for __access_ok()"

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This reverts commit ad5643cf2f ("riscv: Define TASK_SIZE_MAX for
__access_ok()").

This commit changes TASK_SIZE_MAX to be LONG_MAX to optimize access_ok(),
because the previous TASK_SIZE_MAX (default to TASK_SIZE) requires some
computation.

The reasoning was that all user addresses are less than LONG_MAX, and all
kernel addresses are greater than LONG_MAX. Therefore access_ok() can
filter kernel addresses.

Addresses between TASK_SIZE and LONG_MAX are not valid user addresses, but
access_ok() let them pass. That was thought to be okay, because they are
not valid addresses at hardware level.

Unfortunately, one case is missed: get_user_pages_fast() happily accepts
addresses between TASK_SIZE and LONG_MAX. futex(), for instance, uses
get_user_pages_fast(). This causes the problem reported by Robert [1].

Therefore, revert this commit. TASK_SIZE_MAX is changed to the default:
TASK_SIZE.

This unfortunately reduces performance, because TASK_SIZE is more expensive
to compute compared to LONG_MAX. But correctness first, we can think about
optimization later, if required.

Reported-by: <rtm@csail.mit.edu>
Closes: https://lore.kernel.org/linux-riscv/77605.1750245028@localhost/
Signed-off-by: Nam Cao <namcao@linutronix.de>
Cc: stable@vger.kernel.org
Reviewed-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Fixes: ad5643cf2f ("riscv: Define TASK_SIZE_MAX for __access_ok()")
Link: https://lore.kernel.org/r/20250619155858.1249789-1-namcao@linutronix.de
Signed-off-by: Palmer Dabbelt <palmer@dabbelt.com>

...

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Nam Cao 2025-06-19 17:58:58 +02:00 • committed by Palmer Dabbelt

parent b0843f8361

commit 890ba5be63

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arch/riscv/include/asm/pgtable.h

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