[PATCH AUTOSEL 6.7 01/11] sysv: don't call sb_bread() with pointers_lock held

Sasha Levin
2024-03-24T17:06:21Z

From: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>

[ Upstream commit f123dc86388cb669c3d6322702dc441abc35c31e ]

syzbot is reporting sleep in atomic context in SysV filesystem [1], for
sb_bread() is called with rw_spinlock held.

A "write_lock(&pointers_lock) => read_lock(&pointers_lock) deadlock" bug
and a "sb_bread() with write_lock(&pointers_lock)" bug were introduced by
"Replace BKL for chain locking with sysvfs-private rwlock" in Linux 2.5.12.

Then, "[PATCH] err1-40: sysvfs locking fix" in Linux 2.6.8 fixed the
former bug by moving pointers_lock lock to the callers, but instead
introduced a "sb_bread() with read_lock(&pointers_lock)" bug (which made
this problem easier to hit).

Al Viro suggested that why not to do like get_branch()/get_block()/
find_shared() in Minix filesystem does. And doing like that is almost a
revert of "[PATCH] err1-40: sysvfs locking fix" except that get_branch()
from with find_shared() is called without write_lock(&pointers_lock).

Reported-by: syzbot <syzbot+69b40dc5fd40f32c199f@syzkaller.appspotmail.com>
Link: https://syzkaller.appspot.com/bug?extid=69b40dc5fd40f32c199f
Suggested-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Link: https://lore.kernel.org/r/0d195f93-a22a-49a2-0020-103534d6f7f6@I-love.SAKURA.ne.jp
Signed-off-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
fs/sysv/itree.c | 10 ++++------
1 file changed, 4 insertions(+), 6 deletions(-)

diff --git a/fs/sysv/itree.c b/fs/sysv/itree.c
index 725981474e5f9..ff22f39710106 100644
--- a/fs/sysv/itree.c
+++ b/fs/sysv/itree.c
@@ -82,9 +82,6 @@ static inline sysv_zone_t *block_end(struct buffer_head *bh)
return (sysv_zone_t*)((char*)bh->b_data + bh->b_size);
}

-/*
- * Requires read_lock(&pointers_lock) or write_lock(&pointers_lock)
- */
static Indirect *get_branch(struct inode *inode,
int depth,
int offsets[],
@@ -104,15 +101,18 @@ static Indirect *get_branch(struct inode *inode,
bh = sb_bread(sb, block);
if (!bh)
goto failure;
+ read_lock(&pointers_lock);
if (!verify_chain(chain, p))
goto changed;
add_chain(++p, bh, (sysv_zone_t*)bh->b_data + *++offsets);
+ read_unlock(&pointers_lock);
if (!p->key)
goto no_block;
}
return NULL;

changed:
+ read_unlock(&pointers_lock);
brelse(bh);
*err = -EAGAIN;
goto no_block;
@@ -218,9 +218,7 @@ static int get_block(struct inode *inode, sector_t iblock, struct buffer_head *b
goto out;

reread:
- read_lock(&pointers_lock);
partial = get_branch(inode, depth, offsets, chain, &err);
- read_unlock(&pointers_lock);

/* Simplest case - block found, no allocation needed */
if (!partial) {
@@ -290,9 +288,9 @@ static Indirect *find_shared(struct inode *inode,
*top = 0;
for (k = depth; k > 1 && !offsets[k-1]; k--)
;
+ partial = get_branch(inode, k, offsets, chain, &err);

write_lock(&pointers_lock);
- partial = get_branch(inode, k, offsets, chain, &err);
if (!partial)
partial = chain + k-1;
/*
--
2.43.0