There is a potential deadlock reported by syzbot as below:
======================================================
WARNING: possible circular locking dependency detected
6.4.0-next-20230707-syzkaller #0 Not tainted
------------------------------------------------------
syz-executor330/5073 is trying to acquire lock:
ffff8880218527a0 (&mm->mmap_lock){++++}-{3:3}, at: mmap_read_lock_killable include/linux/mmap_lock.h:151 [inline]
ffff8880218527a0 (&mm->mmap_lock){++++}-{3:3}, at: get_mmap_lock_carefully mm/memory.c:5293 [inline]
ffff8880218527a0 (&mm->mmap_lock){++++}-{3:3}, at: lock_mm_and_find_vma+0x369/0x510 mm/memory.c:5344
but task is already holding lock:
ffff888019f760e0 (&sbi->s_lock){+.+.}-{3:3}, at: exfat_iterate+0x117/0xb50 fs/exfat/dir.c:232
which lock already depends on the new lock.
Chain exists of:
&mm->mmap_lock --> mapping.invalidate_lock#3 --> &sbi->s_lock
Possible unsafe locking scenario:
CPU0 CPU1
---- ----
lock(&sbi->s_lock);
lock(mapping.invalidate_lock#3);
lock(&sbi->s_lock);
rlock(&mm->mmap_lock);
Let's try to avoid above potential deadlock condition by moving dir_emit*()
out of sbi->s_lock coverage.
Fixes: ca06197382bd ("exfat: add directory operations")
Cc: [email protected] #v5.7+
Reported-by: [email protected]
Link: https://lore.kernel.org/lkml/[email protected]/T/#u
Signed-off-by: Sungjong Seo <[email protected]>
---
fs/exfat/dir.c | 27 ++++++++++++---------------
1 file changed, 12 insertions(+), 15 deletions(-)
diff --git a/fs/exfat/dir.c b/fs/exfat/dir.c
index 957574180a5e..4e3743341ce7 100644
--- a/fs/exfat/dir.c
+++ b/fs/exfat/dir.c
@@ -214,7 +214,10 @@ static void exfat_free_namebuf(struct exfat_dentry_namebuf *nb)
exfat_init_namebuf(nb);
}
-/* skip iterating emit_dots when dir is empty */
+/*
+ * Before calling dir_emit*(), sbi->s_lock should be released
+ * because page fault can occur in dir_emit*().
+ */
#define ITER_POS_FILLED_DOTS (2)
static int exfat_iterate(struct file *file, struct dir_context *ctx)
{
@@ -229,11 +232,10 @@ static int exfat_iterate(struct file *file, struct dir_context *ctx)
int err = 0, fake_offset = 0;
exfat_init_namebuf(nb);
- mutex_lock(&EXFAT_SB(sb)->s_lock);
cpos = ctx->pos;
if (!dir_emit_dots(file, ctx))
- goto unlock;
+ goto out;
if (ctx->pos == ITER_POS_FILLED_DOTS) {
cpos = 0;
@@ -245,16 +247,18 @@ static int exfat_iterate(struct file *file, struct dir_context *ctx)
/* name buffer should be allocated before use */
err = exfat_alloc_namebuf(nb);
if (err)
- goto unlock;
+ goto out;
get_new:
+ mutex_lock(&EXFAT_SB(sb)->s_lock);
+
if (ei->flags == ALLOC_NO_FAT_CHAIN && cpos >= i_size_read(inode))
goto end_of_dir;
err = exfat_readdir(inode, &cpos, &de);
if (err) {
/*
- * At least we tried to read a sector. Move cpos to next sector
- * position (should be aligned).
+ * At least we tried to read a sector.
+ * Move cpos to next sector position (should be aligned).
*/
if (err == -EIO) {
cpos += 1 << (sb->s_blocksize_bits);
@@ -277,16 +281,10 @@ static int exfat_iterate(struct file *file, struct dir_context *ctx)
inum = iunique(sb, EXFAT_ROOT_INO);
}
- /*
- * Before calling dir_emit(), sb_lock should be released.
- * Because page fault can occur in dir_emit() when the size
- * of buffer given from user is larger than one page size.
- */
mutex_unlock(&EXFAT_SB(sb)->s_lock);
if (!dir_emit(ctx, nb->lfn, strlen(nb->lfn), inum,
(de.attr & ATTR_SUBDIR) ? DT_DIR : DT_REG))
- goto out_unlocked;
- mutex_lock(&EXFAT_SB(sb)->s_lock);
+ goto out;
ctx->pos = cpos;
goto get_new;
@@ -294,9 +292,8 @@ static int exfat_iterate(struct file *file, struct dir_context *ctx)
if (!cpos && fake_offset)
cpos = ITER_POS_FILLED_DOTS;
ctx->pos = cpos;
-unlock:
mutex_unlock(&EXFAT_SB(sb)->s_lock);
-out_unlocked:
+out:
/*
* To improve performance, free namebuf after unlock sb_lock.
* If namebuf is not allocated, this function do nothing
--
2.25.1
2023-07-14 17:43 GMT+09:00, Sungjong Seo <[email protected]>:
> There is a potential deadlock reported by syzbot as below:
>
> ======================================================
> WARNING: possible circular locking dependency detected
> 6.4.0-next-20230707-syzkaller #0 Not tainted
> ------------------------------------------------------
> syz-executor330/5073 is trying to acquire lock:
> ffff8880218527a0 (&mm->mmap_lock){++++}-{3:3}, at: mmap_read_lock_killable
> include/linux/mmap_lock.h:151 [inline]
> ffff8880218527a0 (&mm->mmap_lock){++++}-{3:3}, at: get_mmap_lock_carefully
> mm/memory.c:5293 [inline]
> ffff8880218527a0 (&mm->mmap_lock){++++}-{3:3}, at:
> lock_mm_and_find_vma+0x369/0x510 mm/memory.c:5344
> but task is already holding lock:
> ffff888019f760e0 (&sbi->s_lock){+.+.}-{3:3}, at: exfat_iterate+0x117/0xb50
> fs/exfat/dir.c:232
>
> which lock already depends on the new lock.
>
> Chain exists of:
> &mm->mmap_lock --> mapping.invalidate_lock#3 --> &sbi->s_lock
>
> Possible unsafe locking scenario:
>
> CPU0 CPU1
> ---- ----
> lock(&sbi->s_lock);
> lock(mapping.invalidate_lock#3);
> lock(&sbi->s_lock);
> rlock(&mm->mmap_lock);
>
> Let's try to avoid above potential deadlock condition by moving dir_emit*()
> out of sbi->s_lock coverage.
>
> Fixes: ca06197382bd ("exfat: add directory operations")
> Cc: [email protected] #v5.7+
> Reported-by: [email protected]
> Link:
> https://lore.kernel.org/lkml/[email protected]/T/#u
> Signed-off-by: Sungjong Seo <[email protected]>
Applied it to #dev, Thanks for your patch!