[PATCH 6.1 078/127] exfat: release s_lock before calling dir_emit()

Greg Kroah-Hartman
2023-08-09T10:54:48Z

From: Sungjong Seo <sj1557.seo@samsung.com>

commit ff84772fd45d486e4fc78c82e2f70ce5333543e6 upstream.

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: stable@vger.kernel.org #v5.7+
Reported-by: syzbot+1741a5d9b79989c10bdc@syzkaller.appspotmail.com
Link: https://lore.kernel.org/lkml/00000000000078ee7e060066270b@google.com/T/#u
Tested-by: syzbot+1741a5d9b79989c10bdc@syzkaller.appspotmail.com
Signed-off-by: Sungjong Seo <sj1557.seo@samsung.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
fs/exfat/dir.c | 27 ++++++++++++---------------
1 file changed, 12 insertions(+), 15 deletions(-)

--- a/fs/exfat/dir.c
+++ b/fs/exfat/dir.c
@@ -210,7 +210,10 @@ static void exfat_free_namebuf(struct ex
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)
{
@@ -225,11 +228,10 @@ static int exfat_iterate(struct file *fi
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;
@@ -241,16 +243,18 @@ static int exfat_iterate(struct file *fi
/* 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);
@@ -273,16 +277,10 @@ get_new:
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;

@@ -290,9 +288,8 @@ end_of_dir:
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