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

[Christoph Hellwig <hch@lst.de>]

On Tue, Mar 23, 2021 at 10:24:30PM +0100, 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>

Looks good,

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