Re: [PATCH v2 1/2] target: pscsi: avoid OOM in pscsi_map_sg()

[Lee Duncan <lduncan@suse.com>]

On 3/23/21 2:24 PM, mwilck@suse.com wrote:
> From: Martin Wilck <mwilck@suse.com>
> 
> pscsi_map_sg() uses the variable nr_pages as a hint for bio_kmalloc()
> how many vector elements to allocate. If nr_pages is < BIO_MAX_PAGES,
> it will be reset to 0 after successful allocation of the bio.
> 
> If bio_add_pc_page() fails later for whatever reason, pscsi_map_sg()
> tries to allocate another bio, passing nr_vecs=0. This causes
> bio_add_pc_page() to fail immediately in the next call. pci_map_sg()
> continues to allocate zero-length bios until memory is exhausted and
> the kernel crashes with OOM. This can be easily observed by exporting
> a SATA DVD drive via pscsi. The target crashes as soon as the client
> tries to access the DVD LUN. In the case I analyzed, bio_add_pc_page()
> would fail because the DVD device's max_sectors_kb (128) was
> exceeded.
> 
> Avoid this by simply not resetting nr_pages to 0 after allocating the
> bio. This way, the client receives an IO error when it tries to send
> requests exceeding the devices max_sectors_kb, and eventually gets
> it right. The client must still limit max_sectors_kb e.g. by an udev
> rule if (like in my case) the driver doesn't report valid block
> limits, otherwise it encounters I/O errors.
> 
> Signed-off-by: Martin Wilck <mwilck@suse.com>
> ---
>  drivers/target/target_core_pscsi.c | 1 -
>  1 file changed, 1 deletion(-)
> 
> diff --git a/drivers/target/target_core_pscsi.c b/drivers/target/target_core_pscsi.c
> index 7b1035e..977362d 100644
> --- a/drivers/target/target_core_pscsi.c
> +++ b/drivers/target/target_core_pscsi.c
> @@ -881,7 +881,6 @@ pscsi_map_sg(struct se_cmd *cmd, struct scatterlist *sgl, u32 sgl_nents,
>  			if (!bio) {
>  new_bio:
>  				nr_vecs = bio_max_segs(nr_pages);
> -				nr_pages -= nr_vecs;
>  				/*
>  				 * Calls bio_kmalloc() and sets bio->bi_end_io()
>  				 */
> 

Reviewed-by: Lee Duncan <lduncan@suse.com>