On 2023/5/25 11:16, Tu Jinjiang wrote:
shmem_show_options() uses sbinfo->mpol without adding it's refcnt. This
may lead to race with replacement of the mpol by remount. The execution
sequence is as follows.
CPU0 CPU1
shmem_show_options() shmem_reconfigure()
shmem_show_mpol(seq, sbinfo->mpol) mpol = sbinfo->mpol
mpol_put(mpol)
mpol->mode
The KASAN report is as follows.
BUG: KASAN: slab-use-after-free in shmem_show_options+0x21b/0x340
Read of size 2 at addr ffff888124324004 by task mount/2388
CPU: 2 PID: 2388 Comm: mount Not tainted 6.4.0-rc3-00017-g9d646009f65d-dirty #8
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.13.0-1ubuntu1.1 04/01/2014
Call Trace:
<TASK>
dump_stack_lvl+0x37/0x50
print_report+0xd0/0x620
? shmem_show_options+0x21b/0x340
? __virt_addr_valid+0xf4/0x180
? shmem_show_options+0x21b/0x340
kasan_report+0xb8/0xe0
? shmem_show_options+0x21b/0x340
shmem_show_options+0x21b/0x340
? __pfx_shmem_show_options+0x10/0x10
? strchr+0x2c/0x50
? strlen+0x23/0x40
? seq_puts+0x7d/0x90
show_vfsmnt+0x1e6/0x260
? __pfx_show_vfsmnt+0x10/0x10
? __kasan_kmalloc+0x7f/0x90
seq_read_iter+0x57a/0x740
vfs_read+0x2e2/0x4a0
? __pfx_vfs_read+0x10/0x10
? down_write_killable+0xb8/0x140
? __pfx_down_write_killable+0x10/0x10
? __fget_light+0xa9/0x1e0
? up_write+0x3f/0x80
ksys_read+0xb8/0x150
? __pfx_ksys_read+0x10/0x10
? fpregs_assert_state_consistent+0x55/0x60
? exit_to_user_mode_prepare+0x2d/0x120
do_syscall_64+0x3c/0x90
entry_SYSCALL_64_after_hwframe+0x72/0xdc
</TASK>
Maybe drop the unreliable stack, not mandatory, the patch is look good
to me,
[snip]
To fix the bug, shmem_get_sbmpol() / mpol_put() needs to be called
before / after shmem_show_mpol() call.
Signed-off-by: Tu Jinjiang <tujinjiang@xxxxxxxxxx>
Reviewed-by: Kefeng Wang <wangkefeng.wang@xxxxxxxxxx>
---
mm/shmem.c | 5 ++++-
1 file changed, 4 insertions(+), 1 deletion(-)
diff --git a/mm/shmem.c b/mm/shmem.c
index e40a08c5c6d7..5e54ab5f61f2 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -3726,6 +3726,7 @@ static int shmem_reconfigure(struct fs_context *fc)
static int shmem_show_options(struct seq_file *seq, struct dentry *root)
{
struct shmem_sb_info *sbinfo = SHMEM_SB(root->d_sb);
+ struct mempolicy *mpol;
if (sbinfo->max_blocks != shmem_default_max_blocks())
seq_printf(seq, ",size=%luk",
@@ -3768,7 +3769,9 @@ static int shmem_show_options(struct seq_file *seq, struct dentry *root)
if (sbinfo->huge)
seq_printf(seq, ",huge=%s", shmem_format_huge(sbinfo->huge));
#endif
- shmem_show_mpol(seq, sbinfo->mpol);
+ mpol = shmem_get_sbmpol(sbinfo);
+ shmem_show_mpol(seq, mpol);
+ mpol_put(mpol);
if (sbinfo->noswap)
seq_printf(seq, ",noswap");
return 0;
