On 2022/7/21 23:58, Abhishek Shah wrote:
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.
Hello， could the following changes to avoid the data-race issue？
diff --git a/mm/ksm.c b/mm/ksm.c
index 54f78c9eecae..f072753cbb3a 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -2497,6 +2497,7 @@ int __ksm_enter(struct mm_struct *mm)
{
struct mm_slot *mm_slot;
int needs_wakeup;
+ bool ksm_run_merge;
mm_slot = alloc_mm_slot();
if (!mm_slot)
@@ -2505,6 +2506,10 @@ int __ksm_enter(struct mm_struct *mm)
/* Check ksm_run too? Would need tighter locking */
needs_wakeup = list_empty(&ksm_mm_head.mm_list);
+ mutex_lock(&ksm_thread_mutex);
+ ksm_run_unmerge = !!(ksm_run & KSM_RUN_UNMERGE);
+ mutex_unlock(&ksm_thread_mutex);
+
spin_lock(&ksm_mmlist_lock);
insert_to_mm_slots_hash(mm, mm_slot);
/*
@@ -2517,7 +2522,7 @@ int __ksm_enter(struct mm_struct *mm)
* scanning cursor, otherwise KSM pages in newly forked mms will be
* missed: then we might as well insert at the end of the list.
*/
- if (ksm_run & KSM_RUN_UNMERGE)
+ if (ksm_run_unmerge)
list_add_tail(&mm_slot->mm_list, &ksm_mm_head.mm_list);
else
list_add_tail(&mm_slot->mm_list, &ksm_scan.mm_slot->mm_list);
TraceBUG: 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
---------------------InputsInput 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)
Confirmed.
Thanks for the quick response and apologies for the delay!
Best,
Gabe
On Thu, Jul 21, 2022 at 9:57 PM Kefeng Wang <wangkefeng.wang@xxxxxxxxxx> wrote:
