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f6b1a1cf1c
If the starting position of our insert range happens to be in the hole
between the two ext4_extent_idx, because the lblk of the ext4_extent in
the previous ext4_extent_idx is always less than the start, which leads
to the "extent" variable access across the boundary, the following UAF is
triggered:
==================================================================
BUG: KASAN: use-after-free in ext4_ext_shift_extents+0x257/0x790
Read of size 4 at addr ffff88819807a008 by task fallocate/8010
CPU: 3 PID: 8010 Comm: fallocate Tainted: G E 5.10.0+ #492
Call Trace:
dump_stack+0x7d/0xa3
print_address_description.constprop.0+0x1e/0x220
kasan_report.cold+0x67/0x7f
ext4_ext_shift_extents+0x257/0x790
ext4_insert_range+0x5b6/0x700
ext4_fallocate+0x39e/0x3d0
vfs_fallocate+0x26f/0x470
ksys_fallocate+0x3a/0x70
__x64_sys_fallocate+0x4f/0x60
do_syscall_64+0x33/0x40
entry_SYSCALL_64_after_hwframe+0x44/0xa9
==================================================================
For right shifts, we can divide them into the following situations：
1. When the first ee_block of ext4_extent_idx is greater than or equal to
start, make right shifts directly from the first ee_block.
1) If it is greater than start, we need to continue searching in the
previous ext4_extent_idx.
2) If it is equal to start, we can exit the loop (iterator=NULL).
2. When the first ee_block of ext4_extent_idx is less than start, then
traverse from the last extent to find the first extent whose ee_block
is less than start.
1) If extent is still the last extent after traversal, it means that
the last ee_block of ext4_extent_idx is less than start, that is,
start is located in the hole between idx and (idx+1), so we can
exit the loop directly (break) without right shifts.
2) Otherwise, make right shifts at the corresponding position of the
found extent, and then exit the loop (iterator=NULL).
Fixes:
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.