[PATCHv2 fs/bfs 1/2] bfs: prevent null pointer dereference in bfs_move_block()

[kovalev]

From: Vasiliy Kovalev <kovalev@xxxxxxxxxxxx>

Detect a failed sb_getblk() call (before copying data)
so that null pointer dereferences should not happen any more.

Found by Syzkaller:

KASAN: null-ptr-deref in range [0x0000000000000028-0x000000000000002f]
CPU: 1 PID: 1069 Comm: mark_buffer_dir Tainted: G W 6.10.0-un-def-alt0.rc7
RIP: 0010:bfs_get_block+0x3ab/0xe80 [bfs]
Call Trace:
<TASK>
? show_regs+0x8d/0xa0
? die_addr+0x50/0xd0
? exc_general_protection+0x148/0x220
? asm_exc_general_protection+0x22/0x30
? bfs_get_block+0x3ab/0xe80 [bfs]
? bfs_get_block+0x370/0xe80 [bfs]
? __pfx_bfs_get_block+0x10/0x10 [bfs]
__block_write_begin_int+0x4ae/0x16a0
? __pfx_bfs_get_block+0x10/0x10 [bfs]
? __pfx___block_write_begin_int+0x10/0x10
block_write_begin+0xb5/0x410
? __pfx_bfs_get_block+0x10/0x10 [bfs]
bfs_write_begin+0x32/0xe0 [bfs]
generic_perform_write+0x265/0x610
? __pfx_generic_perform_write+0x10/0x10
? generic_write_checks+0x323/0x4a0
? __pfx_generic_file_write_iter+0x10/0x10
__generic_file_write_iter+0x16a/0x1b0
generic_file_write_iter+0xf0/0x360
? __pfx_generic_file_write_iter+0x10/0x10
vfs_write+0x670/0x1120
? __pfx_vfs_write+0x10/0x10
ksys_write+0x127/0x260
? __pfx_ksys_write+0x10/0x10
do_syscall_64+0x9f/0x190
? __ct_user_enter+0x74/0xc0
? syscall_exit_to_user_mode+0xbb/0x1d0
? do_syscall_64+0xab/0x190
? ct_kernel_exit.isra.0+0xbb/0xe0
? __ct_user_enter+0x74/0xc0
? syscall_exit_to_user_mode+0xbb/0x1d0
? do_syscall_64+0xab/0x190
? ct_kernel_exit.isra.0+0xbb/0xe0
? clear_bhb_loop+0x45/0xa0
? clear_bhb_loop+0x45/0xa0
? clear_bhb_loop+0x45/0xa0
entry_SYSCALL_64_after_hwframe+0x76/0x7e
RIP: 0033:0x7f2bc708ed29

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Reported-by: syzbot+d98fd19acd08b36ff422@xxxxxxxxxxxxxxxxxxxxxxxxx
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Vasiliy Kovalev <kovalev@xxxxxxxxxxxx>
---
 fs/bfs/file.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/fs/bfs/file.c b/fs/bfs/file.c
index 0dceefc54b48a..e99dc8ace2027 100644
--- a/fs/bfs/file.c
+++ b/fs/bfs/file.c
@@ -34,16 +34,22 @@ static int bfs_move_block(unsigned long from, unsigned long to,
 					struct super_block *sb)
 {
 	struct buffer_head *bh, *new;
+	int ret = 0;
 
 	bh = sb_bread(sb, from);
 	if (!bh)
 		return -EIO;
 	new = sb_getblk(sb, to);
+	if (unlikely(!new)) {
+		ret = -EIO;
+		goto out_err_new;
+	}
 	memcpy(new->b_data, bh->b_data, bh->b_size);
 	mark_buffer_dirty(new);
-	bforget(bh);
 	brelse(new);
-	return 0;
+out_err_new:
+	bforget(bh);
+	return ret;
 }
 
 static int bfs_move_blocks(struct super_block *sb, unsigned long start,
-- 
2.33.8