[PATCH 3.18 110/185] fs/proc/proc_sysctl.c: fix potential page fault while unregistering sysctl table

[Greg Kroah-Hartman]

3.18-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Danilo Krummrich <danilokrummrich@xxxxxxxxxxxxx>

[ Upstream commit a0b0d1c345d0317efe594df268feb5ccc99f651e ]

proc_sys_link_fill_cache() does not take currently unregistering sysctl
tables into account, which might result into a page fault in
sysctl_follow_link() - add a check to fix it.

This bug has been present since v3.4.

Link: http://lkml.kernel.org/r/20180228013506.4915-1-danilokrummrich@xxxxxxxxxxxxx
Fixes: 0e47c99d7fe25 ("sysctl: Replace root_list with links between sysctl_table_sets")
Signed-off-by: Danilo Krummrich <danilokrummrich@xxxxxxxxxxxxx>
Acked-by: Kees Cook <keescook@xxxxxxxxxxxx>
Reviewed-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: "Luis R . Rodriguez" <mcgrof@xxxxxxxxxx>
Cc: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
Cc: Alexey Dobriyan <adobriyan@xxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Sasha Levin <alexander.levin@xxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 fs/proc/proc_sysctl.c |    3 +++
 1 file changed, 3 insertions(+)

--- a/fs/proc/proc_sysctl.c
+++ b/fs/proc/proc_sysctl.c
@@ -617,7 +617,10 @@ static bool proc_sys_link_fill_cache(str
 				    struct ctl_table *table)
 {
 	bool ret = true;
+
 	head = sysctl_head_grab(head);
+	if (IS_ERR(head))
+		return false;
 
 	if (S_ISLNK(table->mode)) {
 		/* It is not an error if we can not follow the link ignore it */