sysv: don't call sb_bread() with pointers_lock held

[ 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>

diff --git a/fs/sysv/itree.c b/fs/sysv/itree.c
index 410ab2a44d2f604d22d7e805317cfa616ca5d191..19bcb51a220366631b9b89c0dfa0e86ddada208c 100644 (file)
--- a/fs/sysv/itree.c
+++ b/fs/sysv/itree.c
@@ -83,9 +83,6 @@ 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[],
@@ -105,15 +102,18 @@ 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;
@@ -219,9 +219,7 @@ 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) {
@@ -291,9 +289,9 @@ 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;
        /*