Race in mm/ksm.c

[Abhishek Shah <abhishek.shah@columbia.edu>]

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Dear Kernel Maintainers,

We found a race in mm/ksm.c. During the execution of the function
*__ksm_run* which uses variable *ksm_run* to decide the list insertion
point, the variable *ksm_run* can be concurrently modified in the function
*run_store*, which we thought could be undesirable since “KSM pages in
newly forked mms can be missed” (See comment here:
https://elixir.bootlin.com/linux/v5.18-rc5/source/mm/ksm.c#L2498). We would
also like your thoughts on the security impact given it is a TOCTOU bug.

We provide more details below including the trace and reproducing
test cases.


*Trace*
BUG: KCSAN: data-race in __ksm_enter / run_store
write to 0xffffffff881edae0 of 8 bytes by task 6542 on cpu 0:
 run_store+0x19a/0x2d0 mm/ksm.c:2897
 kobj_attr_store+0x44/0x60 lib/kobject.c:824
 sysfs_kf_write+0x16f/0x1a0 fs/sysfs/file.c:136
 kernfs_fop_write_iter+0x2ae/0x370 fs/kernfs/file.c:291
 call_write_iter include/linux/fs.h:2050 [inline]
 new_sync_write fs/read_write.c:504 [inline]
 vfs_write+0x779/0x900 fs/read_write.c:591
 ksys_write+0xde/0x190 fs/read_write.c:644
 __do_sys_write fs/read_write.c:656 [inline]
 __se_sys_write fs/read_write.c:653 [inline]
 __x64_sys_write+0x43/0x50 fs/read_write.c:653
 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x3d/0x90 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x44/0xae

read to 0xffffffff881edae0 of 8 bytes by task 6541 on cpu 1:
 __ksm_enter+0x114/0x260 mm/ksm.c:2501
 ksm_madvise+0x291/0x350 mm/ksm.c:2451
 madvise_vma_behavior mm/madvise.c:1039 [inline]
 madvise_walk_vmas mm/madvise.c:1221 [inline]
 do_madvise+0x656/0xeb0 mm/madvise.c:1399
 __do_sys_madvise mm/madvise.c:1412 [inline]
 __se_sys_madvise mm/madvise.c:1410 [inline]
 __x64_sys_madvise+0x64/0x70 mm/madvise.c:1410
 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x3d/0x90 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x44/0xae

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 6541 Comm: syz-executor2-n Not tainted 5.18.0-rc5+ #107
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.15.0-1
04/01/2014
---------------------
*Inputs *
Input CPU 0:
r0 = openat$sysctl(0xffffff9c,
&(0x7f0000000100)='/sys/kernel/mm/ksm/run\x00', 0x1, 0x0)
write$sysctl(r0, &(0x7f0000000000)='2\x00', 0x2)

Input CPU 1:
madvise(&(0x7f0000ffc000/0x4000)=nil, 0x4000, 0xc)
mlock2(&(0x7f0000ffe000/0x2000)=nil, 0x2000, 0x0)
madvise(&(0x7f0000ffd000/0x3000)=nil, 0x3000, 0x12)
clone(0x0, 0x0, 0x0, 0x0, 0x0)

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