[PATCH] io_uring/msg_ring: Fix NULL pointer dereference in io_msg_send_fd()

Harshit Mogalapalli
2022-10-19T17:12:37Z

Syzkaller produced the below call trace:

BUG: KASAN: null-ptr-deref in io_msg_ring+0x3cb/0x9f0
Write of size 8 at addr 0000000000000070 by task repro/16399

CPU: 0 PID: 16399 Comm: repro Not tainted 6.1.0-rc1 #28
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.11.0-2.el7
Call Trace:
<TASK>
dump_stack_lvl+0xcd/0x134
? io_msg_ring+0x3cb/0x9f0
kasan_report+0xbc/0xf0
? io_msg_ring+0x3cb/0x9f0
kasan_check_range+0x140/0x190
io_msg_ring+0x3cb/0x9f0
? io_msg_ring_prep+0x300/0x300
io_issue_sqe+0x698/0xca0
io_submit_sqes+0x92f/0x1c30
__do_sys_io_uring_enter+0xae4/0x24b0
....
RIP: 0033:0x7f2eaf8f8289
RSP: 002b:00007fff40939718 EFLAGS: 00000246 ORIG_RAX: 00000000000001aa
RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007f2eaf8f8289
RDX: 0000000000000000 RSI: 0000000000006f71 RDI: 0000000000000004
RBP: 00007fff409397a0 R08: 0000000000000000 R09: 0000000000000039
R10: 0000000000000000 R11: 0000000000000246 R12: 00000000004006d0
R13: 00007fff40939880 R14: 0000000000000000 R15: 0000000000000000
</TASK>
Kernel panic - not syncing: panic_on_warn set ...

We don't have a NULL check on file_ptr in io_msg_send_fd() function,
so when file_ptr is NUL src_file is also NULL and get_file()
dereferences a NULL pointer and leads to above crash.

Add a NULL check to fix this issue.

Fixes: e6130eba8a84 ("io_uring: add support for passing fixed file descriptors")
Reported-by: syzkaller <syzkaller@googlegroups.com>
Signed-off-by: Harshit Mogalapalli <harshit.m.mogalapalli@oracle.com>
---
I am not completely sure whether to place the NULL check on file_ptr
which i did in this case as file_ptr is NULL, or the masked src_file.

Similar checks are present in other files, io_uring/filetable.c has NULL
check before masking and io_uring/cancel.c has NULL check after masking
with FFS_MASK.
---
io_uring/msg_ring.c | 3 +++
1 file changed, 3 insertions(+)

diff --git a/io_uring/msg_ring.c b/io_uring/msg_ring.c
index 4a7e5d030c78..90d2fc6fd80e 100644
--- a/io_uring/msg_ring.c
+++ b/io_uring/msg_ring.c
@@ -95,6 +95,9 @@ static int io_msg_send_fd(struct io_kiocb *req, unsigned int issue_flags)

msg->src_fd = array_index_nospec(msg->src_fd, ctx->nr_user_files);
file_ptr = io_fixed_file_slot(&ctx->file_table, msg->src_fd)->file_ptr;
+ if (!file_ptr)
+ goto out_unlock;
+
src_file = (struct file *) (file_ptr & FFS_MASK);
get_file(src_file);

--
2.37.1