[patch] groups: integer underflow in groups_alloc()

[Dan Carpenter <dan.carpenter@oracle.com>]

This is called from rsc_parse() with a use controlled value.  Say for
example that "gidsetsize" is negative, then we could end up allocating
less than sizeof(struct group_info) leading to memory corruption.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
---
I copied the NGROUPS_MAX limit from the surrounding code, I'm not
absolutely that it's the correct limit to use.

diff --git a/kernel/groups.c b/kernel/groups.c
index 664411f..e9341b3 100644
--- a/kernel/groups.c
+++ b/kernel/groups.c
@@ -18,6 +18,9 @@ struct group_info *groups_alloc(int gidsetsize)
 	int nblocks;
 	int i;
 
+	if ((unsigned)gidsetsize > NGROUPS_MAX)
+		return NULL;
+
 	nblocks = (gidsetsize + NGROUPS_PER_BLOCK - 1) / NGROUPS_PER_BLOCK;
 	/* Make sure we always allocate at least one indirect block pointer */
 	nblocks = nblocks ? : 1;