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

[mwilck@suse.com]

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()
 				 */
-- 
2.26.2