Re: [PATCH] io_uring/rsrc: fix slab-out-of-bounds in io_buffer_register_bvec

[Ming Lei <ming.lei@redhat.com>]

On Wed, Dec 17, 2025 at 09:04:01PM +0000, veygax wrote:
> From: Evan Lambert <veyga@veygax.dev>
> 
> The function io_buffer_register_bvec() calculates the allocation size
> for the io_mapped_ubuf based on blk_rq_nr_phys_segments(rq). This
> function calculates the number of scatter-gather elements after megine
> physically contiguous pages.
> 
> However, the subsequent loop uses rq_for_each_bvec() to populate the
> array, which iterates over every individual bio_vec in the request,
> regardless of physical contiguity.
> 
> If a request has multiple bio_vec entries that are physically
> contiguous, blk_rq_nr_phys_segments() returns a value smaller than
> the total number of bio_vecs. This leads to a slab-out-of-bounds write.
> 
> The path is reachable from userspace via the ublk driver when a server
> issues a UBLK_IO_REGISTER_IO_BUF command. This requires the
> UBLK_F_SUPPORT_ZERO_COPY flag which is protected by CAP_NET_ADMIN.
> 
> Fix this by calculating the total number of bio_vecs by iterating
> over the request's bios and summing their bi_vcnt.
> 
> KASAN report:
> 
> [18:01:50] BUG: KASAN: slab-out-of-bounds in io_buffer_register_bvec+0x813/0xb80
> [18:01:50] Write of size 8 at addr ffff88800223b238 by task kunit_try_catch/27

Can you share the test case so that we can understand why page isn't merged
to last bvec? Maybe there is chance to improve block layer(bio add page
related code)

> [18:01:50]
> [18:01:50] CPU: 0 UID: 0 PID: 27 Comm: kunit_try_catch Tainted: G                 N  6.19.0-rc1-g346af1a0c65a-dirty #44 PREEMPT(none)
> [18:01:50] Tainted: [N]=TEST
> [18:01:50] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 0.1 11/11/2019
> [18:01:50] Call Trace:
> [18:01:50]  <TASK>
> [18:01:50]  dump_stack_lvl+0x4d/0x70
> [18:01:50]  print_report+0x151/0x4c0
> [18:01:50]  ? __pfx__raw_spin_lock_irqsave+0x10/0x10
> [18:01:50]  ? io_buffer_register_bvec+0x813/0xb80
> [18:01:50]  kasan_report+0xec/0x120
> [18:01:50]  ? io_buffer_register_bvec+0x813/0xb80
> [18:01:50]  io_buffer_register_bvec+0x813/0xb80
> [18:01:50]  io_buffer_register_bvec_overflow_test+0x4e6/0x9b0
> [18:01:50]  ? __pfx_io_buffer_register_bvec_overflow_test+0x10/0x10
> [18:01:50]  ? __pfx_pick_next_task_fair+0x10/0x10
> [18:01:50]  ? _raw_spin_lock+0x7e/0xd0
> [18:01:50]  ? finish_task_switch.isra.0+0x19a/0x650
> [18:01:50]  ? __pfx_read_tsc+0x10/0x10
> [18:01:50]  ? ktime_get_ts64+0x79/0x240
> [18:01:50]  kunit_try_run_case+0x19b/0x2c0
> [18:01:50]  ? __pfx_kunit_try_run_case+0x10/0x10
> [18:01:50]  ? __pfx_kunit_generic_run_threadfn_adapter+0x10/0x10
> [18:01:50]  kunit_generic_run_threadfn_adapter+0x80/0xf0
> [18:01:50]  kthread+0x323/0x670
> [18:01:50]  ? __pfx_kthread+0x10/0x10
> [18:01:50]  ? __pfx__raw_spin_lock_irq+0x10/0x10
> [18:01:50]  ? __pfx_kthread+0x10/0x10
> [18:01:50]  ret_from_fork+0x329/0x420
> [18:01:50]  ? __pfx_ret_from_fork+0x10/0x10
> [18:01:50]  ? __switch_to+0xa0f/0xd40
> [18:01:50]  ? __pfx_kthread+0x10/0x10
> [18:01:50]  ret_from_fork_asm+0x1a/0x30
> [18:01:50]  </TASK>
> [18:01:50]
> [18:01:50] Allocated by task 27:
> [18:01:50]  kasan_save_stack+0x30/0x50
> [18:01:50]  kasan_save_track+0x14/0x30
> [18:01:50]  __kasan_kmalloc+0x7f/0x90
> [18:01:50]  io_cache_alloc_new+0x35/0xc0
> [18:01:50]  io_buffer_register_bvec+0x196/0xb80
> [18:01:50]  io_buffer_register_bvec_overflow_test+0x4e6/0x9b0
> [18:01:50]  kunit_try_run_case+0x19b/0x2c0
> [18:01:50]  kunit_generic_run_threadfn_adapter+0x80/0xf0
> [18:01:50]  kthread+0x323/0x670
> [18:01:50]  ret_from_fork+0x329/0x420
> [18:01:50]  ret_from_fork_asm+0x1a/0x30
> [18:01:50]
> [18:01:50] The buggy address belongs to the object at ffff88800223b000
> [18:01:50]  which belongs to the cache kmalloc-1k of size 1024
> [18:01:50] The buggy address is located 0 bytes to the right of
> [18:01:50]  allocated 568-byte region [ffff88800223b000, ffff88800223b238)
> [18:01:50]
> [18:01:50] The buggy address belongs to the physical page:
> [18:01:50] page: refcount:0 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x2238
> [18:01:50] head: order:2 mapcount:0 entire_mapcount:0 nr_pages_mapped:0 pincount:0
> [18:01:50] flags: 0x4000000000000040(head|zone=1)
> [18:01:50] page_type: f5(slab)
> [18:01:50] raw: 4000000000000040 ffff888001041dc0 dead000000000122 0000000000000000
> [18:01:50] raw: 0000000000000000 0000000080080008 00000000f5000000 0000000000000000
> [18:01:50] head: 4000000000000040 ffff888001041dc0 dead000000000122 0000000000000000
> [18:01:50] head: 0000000000000000 0000000080080008 00000000f5000000 0000000000000000
> [18:01:50] head: 4000000000000002 ffffea0000088e01 00000000ffffffff 00000000ffffffff
> [18:01:50] head: 0000000000000000 0000000000000000 00000000ffffffff 0000000000000000
> [18:01:50] page dumped because: kasan: bad access detected
> [18:01:50]
> [18:01:50] Memory state around the buggy address:
> [18:01:50]  ffff88800223b100: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
> [18:01:50]  ffff88800223b180: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
> [18:01:50] >ffff88800223b200: 00 00 00 00 00 00 00 fc fc fc fc fc fc fc fc fc
> [18:01:50]                                         ^
> [18:01:50]  ffff88800223b280: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
> [18:01:50]  ffff88800223b300: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
> [18:01:50] ==================================================================
> [18:01:50] Disabling lock debugging due to kernel taint
> 
> Fixes: 27cb27b6d5ea ("io_uring: add support for kernel registered bvecs")
> Signed-off-by: Evan Lambert <veyga@veygax.dev>
> ---
>  io_uring/rsrc.c | 11 ++++++-----
>  1 file changed, 6 insertions(+), 5 deletions(-)
> 
> diff --git a/io_uring/rsrc.c b/io_uring/rsrc.c
> index a63474b331bf..7602b71543e0 100644
> --- a/io_uring/rsrc.c
> +++ b/io_uring/rsrc.c
> @@ -946,6 +946,7 @@ int io_buffer_register_bvec(struct io_uring_cmd *cmd, struct request *rq,
>  	struct io_mapped_ubuf *imu;
>  	struct io_rsrc_node *node;
>  	struct bio_vec bv;
> +	struct bio *bio;
>  	unsigned int nr_bvecs = 0;
>  	int ret = 0;
>  
> @@ -967,11 +968,10 @@ int io_buffer_register_bvec(struct io_uring_cmd *cmd, struct request *rq,
>  		goto unlock;
>  	}
>  
> -	/*
> -	 * blk_rq_nr_phys_segments() may overestimate the number of bvecs
> -	 * but avoids needing to iterate over the bvecs
> -	 */
> -	imu = io_alloc_imu(ctx, blk_rq_nr_phys_segments(rq));
> +	__rq_for_each_bio(bio, rq)
> +		nr_bvecs += bio->bi_vcnt;

This way is wrong, bio->bi_vcnt can't be trusted for this purpose, you may
have to use rq_for_each_bvec() for calculating the real nr_bvecs.


Thanks, 
Ming