CVE: CVE-2025-38241
Published: 2025-07-09
Description: In the Linux kernel, the following vulnerability has been resolved:

mm/shmem, swap: fix softlockup with mTHP swapin

Following softlockup can be easily reproduced on my test machine with:

echo always > /sys/kernel/mm/transparent_hugepage/hugepages-64kB/enabled
swapon /dev/zram0 # zram0 is a 48G swap device
mkdir -p /sys/fs/cgroup/memory/test
echo 1G > /sys/fs/cgroup/test/memory.max
echo $BASHPID > /sys/fs/cgroup/test/cgroup.procs
while true; do
    dd if=/dev/zero of=/tmp/test.img bs=1M count=5120
    cat /tmp/test.img > /dev/null
    rm /tmp/test.img
done

Then after a while:
watchdog: BUG: soft lockup - CPU#0 stuck for 763s! [cat:5787]
Modules linked in: zram virtiofs
CPU: 0 UID: 0 PID: 5787 Comm: cat Kdump: loaded Tainted: G             L      6.15.0.orig-gf3021d9246bc-dirty #118 PREEMPT(voluntary)·
Tainted: [L]=SOFTLOCKUP
Hardware name: Red Hat KVM/RHEL-AV, BIOS 0.0.0 02/06/2015
RIP: 0010:mpol_shared_policy_lookup+0xd/0x70
Code: e9 b8 b4 ff ff 31 c0 c3 cc cc cc cc 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 66 0f 1f 00 0f 1f 44 00 00 41 54 55 53 <48> 8b 1f 48 85 db 74 41 4c 8d 67 08 48 89 fb 48 89 f5 4c 89 e7 e8
RSP: 0018:ffffc90002b1fc28 EFLAGS: 00000202
RAX:[PII_REDACTED]1c20ca RBX:[PII_REDACTED]724e1e RCX: [PII_REDACTED]
RDX: ffff888118e214c8 RSI:[PII_REDACTED]057d42 RDI: ffff888118e21518
RBP:[PII_REDACTED]02bec8 R08: [PII_REDACTED] R09: [PII_REDACTED]
R10:[PII_REDACTED]000bf4 R11: [PII_REDACTED] R12: [PII_REDACTED]
R13:[PII_REDACTED]1c20ca R14:[PII_REDACTED]1c20ca R15: [PII_REDACTED]
FS:  00007f03f995c740(0000) GS:ffff88a07ad9a000(0000) knlGS:[PII_REDACTED]
CS:  0010 DS: 0000 ES: 0000 CR0: [PII_REDACTED]
CR2: 00007f03f98f1000 CR3: [PII_REDACTED] CR4:[PII_REDACTED]770eb0
DR0: [PII_REDACTED] DR1: [PII_REDACTED] DR2: [PII_REDACTED]
DR3: [PII_REDACTED] DR6: 00000000fffe0ff0 DR7: [PII_REDACTED]
PKRU: 55555554
Call Trace:
 <TASK>
 shmem_alloc_folio+0x31/0xc0
 shmem_swapin_folio+0x309/0xcf0
 ? filemap_get_entry+0x117/0x1e0
 ? xas_load+0xd/0xb0
 ? filemap_get_entry+0x101/0x1e0
 shmem_get_folio_gfp+0x2ed/0x5b0
 shmem_file_read_iter+0x7f/0x2e0
 vfs_read+0x252/0x330
 ksys_read+0x68/0xf0
 do_syscall_64+0x4c/0x1c0
 entry_SYSCALL_64_after_hwframe+0x76/0x7e
RIP: 0033:0x7f03f9a46991
Code: 00 48 8b 15 81 14 10 00 f7 d8 64 89 02 b8 ff ff ff ff eb bd e8 20 ad 01 00 f3 0f 1e fa 80 3d 35 97 10 00 00 74 13 31 c0 0f 05 <48> 3d 00 f0 ff ff 77 4f c3 66 0f 1f 44 00 00 55 48 89 e5 48 83 ec
RSP: 002b:00007fff3c52bd28 EFLAGS: 00000246 ORIG_RAX: [PII_REDACTED]
RAX: ffffffffffffffda RBX: [PII_REDACTED] RCX: 00007f03f9a46991
RDX: [PII_REDACTED] RSI: 00007f03f98ba000 RDI: [PII_REDACTED]
RBP: 00007fff3c52bd50 R08: [PII_REDACTED] R09: 00007f03f9b9a380
R10: [PII_REDACTED] R11: [PII_REDACTED] R12: [PII_REDACTED]
R13: 00007f03f98ba000 R14: [PII_REDACTED] R15: [PII_REDACTED]
 </TASK>

The reason is simple, readahead brought some order 0 folio in swap cache,
and the swapin mTHP folio being allocated is in conflict with it, so
swapcache_prepare fails and causes shmem_swap_alloc_folio to return
-EEXIST, and shmem simply retries again and again causing this loop.

Fix it by applying a similar fix for anon mTHP swapin.

The performance change is very slight, time of swapin 10g zero folios
with shmem (test for 12 times):
Before:  2.47s
After:   2.48s

[kasong@tencent.com: add comment]
CVSS Score: 5.5 (MEDIUM)
CWE: CWE-667
References: https://git.kernel.org/stable/c/1283dfc1e0cd52cf525c2cb1b59a6f9183aab7ca; https://git.kernel.org/stable/c/a05dd8ae5cbb1cb45f349922cfea4f548a5e5d6f