fs/proc/proc_sysctl.c: fix potential page fault while unregistering sysctl table

[ 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@dk-develop.de
Fixes: 0e47c99d7fe25 ("sysctl: Replace root_list with links between sysctl_table_sets")
Signed-off-by: Danilo Krummrich <danilokrummrich@dk-develop.de>
Acked-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: "Luis R . Rodriguez" <mcgrof@kernel.org>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/fs/proc/proc_sysctl.c b/fs/proc/proc_sysctl.c
index d4e37acd48217dcb1090a1ee68bb4ec4cb7226e5..847f23420b407ea4e7939efd5caf324d987ef5c3 100644 (file)
--- a/fs/proc/proc_sysctl.c
+++ b/fs/proc/proc_sysctl.c
@@ -660,7 +660,10 @@ static bool proc_sys_link_fill_cache(struct file *file,
                                    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 */