[PATCH] amd64-agp: fix arbitrary kernel memory writes

[Young Xiao <92siuyang@gmail.com>]

pg_start is copied from userspace on AGPIOC_BIND and AGPIOC_UNBIND ioctl
cmds of agp_ioctl() and passed to agpioc_bind_wrap().  As said in the
comment, (pg_start + mem->page_count) may wrap in case of AGPIOC_BIND,
and it is not checked at all in case of AGPIOC_UNBIND.  As a result, user
with sufficient privileges (usually "video" group) may generate either
local DoS or privilege escalation.

See commit 194b3da873fd ("agp: fix arbitrary kernel memory writes")
for details.

Signed-off-by: Young Xiao <92siuyang@gmail.com>
---
 drivers/char/agp/amd64-agp.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/char/agp/amd64-agp.c b/drivers/char/agp/amd64-agp.c
index c69e39f..5daa0e3 100644
--- a/drivers/char/agp/amd64-agp.c
+++ b/drivers/char/agp/amd64-agp.c
@@ -60,7 +60,8 @@ static int amd64_insert_memory(struct agp_memory *mem, off_t pg_start, int type)
 
 	/* Make sure we can fit the range in the gatt table. */
 	/* FIXME: could wrap */
-	if (((unsigned long)pg_start + mem->page_count) > num_entries)
+	if (((pg_start + mem->page_count) > num_entries) ||
+	    ((pg_start + mem->page_count) < pg_start))
 		return -EINVAL;
 
 	j = pg_start;
-- 
2.7.4