The userspace can configure a loop using an ioctl call, wherein
a configuration of type loop_config is passed (see lo_ioctl()'s
case on line 1550 of drivers/block/loop.c). This proceeds to call
loop_configure() which in turn calls loop_set_status_from_info()
(see line 1050 of loop.c), passing &config->info which is of type
loop_info64*. This function then sets the appropriate values, like
the offset.

loop_device has lo_offset of type loff_t (see line 52 of loop.c),
which is typdef-chained to long long, whereas loop_info64 has
lo_offset of type __u64 (see line 56 of include/uapi/linux/loop.h).

The function directly copies offset from info to the device as
follows (See line 980 of loop.c):
	lo->lo_offset = info->lo_offset;

This results in an overflow, which triggers a warning in iomap_iter()
due to a call to iomap_iter_done() which has:
	WARN_ON_ONCE(iter->iomap.offset > iter->pos);

Thus, check for negative value during loop_set_status_from_info().

Bug report: https://syzkaller.appspot.com/bug?id=c620fe14aac810396d3c3edc9ad73848bf69a29e
Reported-and-tested-by: syzbot+a8e049cd3abd342936b6@syzkaller.appspotmail.com
Cc: stable@vger.kernel.org
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Siddh Raman Pant <code@siddh.me>
---
Changes since v1:
- Do not break userspace API, so check loop_device for overflow.
- Use EOVERFLOW instead of EINVAL.

 drivers/block/loop.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index e3c0ba93c1a3..ad92192c7d61 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -979,6 +979,11 @@ loop_set_status_from_info(struct loop_device *lo,
 
 	lo->lo_offset = info->lo_offset;
 	lo->lo_sizelimit = info->lo_sizelimit;
+
+	/* loff_t vars have been assigned __u64 */
+	if (lo->lo_offset < 0 || lo->lo_sizelimit < 0)
+		return -EOVERFLOW;
+
 	memcpy(lo->lo_file_name, info->lo_file_name, LO_NAME_SIZE);
 	lo->lo_file_name[LO_NAME_SIZE-1] = 0;
 	lo->lo_flags = info->lo_flags;
-- 
2.35.1

On Tue, 23 Aug 2022 21:38:10 +0530, Siddh Raman Pant via Linux-kernel-mentees wrote:
> The userspace can configure a loop using an ioctl call, wherein
> a configuration of type loop_config is passed (see lo_ioctl()'s
> case on line 1550 of drivers/block/loop.c). This proceeds to call
> loop_configure() which in turn calls loop_set_status_from_info()
> (see line 1050 of loop.c), passing &config->info which is of type
> loop_info64*. This function then sets the appropriate values, like
> the offset.
> 
> [...]

Applied, thanks!

[1/1] loop: Check for overflow while configuring loop
      commit: f11ebc7347340d291ba032a3872e40d3283fc351

Best regards,
-- 
Jens Axboe

On Tue, 23 Aug 2022 21:46:46 +0530  Jens Axboe  wrote:
> Applied, thanks!
> 
> [1/1] loop: Check for overflow while configuring loop
>       commit: f11ebc7347340d291ba032a3872e40d3283fc351

Thanks!

Though, it seems that you have used the mailing list's address
(linux-kernel-mentees@lists.linuxfoundation.org) in the author
field instead of the Signed-off email.

Thanks,
Siddh

On 8/24/22 2:12 AM, Siddh Raman Pant wrote:
> On Tue, 23 Aug 2022 21:46:46 +0530  Jens Axboe  wrote:
>> Applied, thanks!
>>
>> [1/1] loop: Check for overflow while configuring loop
>>       commit: f11ebc7347340d291ba032a3872e40d3283fc351
> 
> Thanks!
> 
> Though, it seems that you have used the mailing list's address
> (linux-kernel-mentees@lists.linuxfoundation.org) in the author
> field instead of the Signed-off email.

Weird! I'll let Konstantin know about this one, I've manually
fixed it up.

-- 
Jens Axboe

Looks good:

Reviewed-by: Christoph Hellwig <hch@lst.de>