[PATCH v3 1/2] bfs: prevent null pointer dereference in bfs_move_block()

[kovalev@altlinux.org]

From: Vasiliy Kovalev <kovalev@altlinux.org>

Detect a failed sb_getblk() call (before copying data)
so that null pointer dereferences should not happen any more.
We also decrement (brelse) the bh counter in this case.

Found when launching the reproducer generated by Syzkaller:

KASAN: null-ptr-deref in range [0x0000000000000028-0x000000000000002f]
Comm: mark_buffer_dir Tainted: G  W  6.1.105-un-def-alt1.kasan #1
RIP: 0010:bfs_get_block+0x35b/0xdf0 (fs/bfs/file.c:42) [bfs]
Call Trace:
<TASK>
 __die_body.cold (arch/x86/kernel/dumpstack.c:478)
 die_addr (arch/x86/kernel/dumpstack.c:462)
 exc_general_protection (arch/x86/kernel/traps.c:787)
 asm_exc_general_protection (./arch/x86/include/asm/idtentry.h:564)
 __getblk_gfp (fs/buffer.c:1335)
 bfs_get_block (fs/bfs/file.c:42) bfs
 bfs_get_block (fs/bfs/file.c:56) bfs
 bfs_get_block (fs/bfs/file.c:125) bfs
 bfs_write_begin (fs/bfs/file.c:66) bfs
 __block_write_begin_int (fs/buffer.c:1991)
 bfs_write_begin (fs/bfs/file.c:66) bfs
 invalidate_bh_lrus_cpu (fs/buffer.c:1955)
 fault_in_readable (mm/gup.c:1898)
 PageHeadHuge (mm/hugetlb.c:2123)
 bfs_write_begin (fs/bfs/file.c:66) bfs
 block_write_begin (fs/buffer.c:2103)
 bfs_write_begin (fs/bfs/file.c:178) bfs
 generic_perform_write (mm/filemap.c:3817)
 generic_file_readonly_mmap (mm/filemap.c:3781)
 new_inode (fs/inode.c:2126)
 generic_write_checks (fs/read_write.c:1687)
 __generic_file_write_iter (mm/filemap.c:3946)
 generic_file_write_iter (./include/linux/fs.h:763 mm/filemap.c:3978)
 vfs_write (./include/linux/fs.h:2265 fs/read_write.c:491)
 kernel_write (fs/read_write.c:565)
 __fget_files (fs/file.c:918)
 ksys_write (fs/read_write.c:638)

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Reported-by: syzbot+d98fd19acd08b36ff422@syzkaller.appspotmail.com
Link: https://syzkaller.appspot.com/bug?extid=d98fd19acd08b36ff422
Link: https://syzkaller.appspot.com/text?tag=ReproC&x=16515ba3e80000
Cc: stable@vger.kernel.org
Signed-off-by: Vasiliy Kovalev <kovalev@altlinux.org>
---
 fs/bfs/file.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/fs/bfs/file.c b/fs/bfs/file.c
index 57ae5ee6deec12..23773e62994024 100644
--- a/fs/bfs/file.c
+++ b/fs/bfs/file.c
@@ -39,6 +39,10 @@ static int bfs_move_block(unsigned long from, unsigned long to,
 	if (!bh)
 		return -EIO;
 	new = sb_getblk(sb, to);
+	if (!new) {
+		brelse(bh);
+		return -EIO;
+	}
 	memcpy(new->b_data, bh->b_data, bh->b_size);
 	mark_buffer_dirty(new);
 	bforget(bh);
-- 
2.33.8