The blk_rq_map_user_iov function in block/blk-map.c in the Linux kernel
before 4.8.14 does not properly restrict the type of iterator, which allows
local users to read or write to arbitrary kernel memory locations or cause
a denial of service (use-after-free) by leveraging access to a /dev/sg
I've manually tested these patches using the syzkaller reproducer and
sg_dd from sg3-utils inside of a VM with a SCSI drive attached.
The 'reserved' page array is used as a short-cut for mapping data,
saving us to allocate pages per request. However, the 'reserved' array
is only capable of holding one request, so this patch introduces a mutex
for protect 'sg_fd' against concurrent accesses.
Once the reserved page array is unused we can reset the 'res_in_use'
state; here we can do a lazy update without holding the mutex as we only
need to check against concurrent access, not concurrent release.
Commit 1bc0eb044615 ("scsi: sg: protect accesses to 'reserved' page
array") adds needed concurrency protection for the "reserve" buffer.
Some checks that are initially made outside the lock are replicated once
the lock is taken to ensure the checks and resulting decisions are made
using consistent state.
The check that a request with flag SG_FLAG_MMAP_IO set fits in the
reserve buffer also needs to be performed again under the lock to ensure
the reserve buffer length compared against matches the value in effect
when the request is linked to the reserve buffer. An -ENOMEM should be
returned in this case, instead of switching over to an indirect buffer
as for non-MMAP_IO requests.