[PATCH] resource: Fix the case of null pointer access

From: Vivek Goyal
Date: Tue Aug 12 2014 - 10:47:44 EST

Richard and Daniel reported that UML is broken due to changes to resource
traversal functions. Problem is that iomem_resource.child can be null
and new code does not consider that possibility. Old code used a for loop
and that loop will not even execute if p was null.

Revert back to for() loop logic and bail out if p is null.

I also moved sibling_only check out of resource_lock. There is no
reason to keep it inside the lock.

Following is backtrace of the UML crash.

RIP: 0033:[<0000000060039b9f>]
RSP: 0000000081459da0 EFLAGS: 00010202
RAX: 0000000000000000 RBX: 00000000219b3fff RCX: 000000006010d1d9
RDX: 0000000000000001 RSI: 00000000602dfb94 RDI: 0000000081459df8
RBP: 0000000081459de0 R08: 00000000601b59f4 R09: ffffffff0000ff00
R10: ffffffff0000ff00 R11: 0000000081459e88 R12: 0000000081459df8
R13: 00000000219b3fff R14: 00000000602dfb94 R15: 0000000000000000
Kernel panic - not syncing: Segfault with no mm
CPU: 0 PID: 1 Comm: swapper Not tainted 3.16.0-10454-g58d08e3 #13
00000000 000080d0 81459df0 219b3fff
81459e70 6010d1d9 ffffffff 6033e010
81459e50 6003a269 81459e30 00000000
Call Trace:
[<6010d1d9>] ? kclist_add_private+0x0/0xe7
[<6003a269>] walk_system_ram_range+0x61/0xb7
[<6000e859>] ? proc_kcore_init+0x0/0xf1
[<6010d574>] kcore_update_ram+0x4c/0x168
[<6010d72e>] ? kclist_add+0x0/0x2e
[<6000e943>] proc_kcore_init+0xea/0xf1
[<6000e859>] ? proc_kcore_init+0x0/0xf1
[<6000e859>] ? proc_kcore_init+0x0/0xf1
[<600189f0>] do_one_initcall+0x13c/0x204
[<6004ca46>] ? parse_args+0x1df/0x2e0
[<6004c82d>] ? parameq+0x0/0x3a
[<601b5990>] ? strcpy+0x0/0x18
[<60001e1a>] kernel_init_freeable+0x240/0x31e
[<6026f1c0>] kernel_init+0x12/0x148
[<60019fad>] new_thread_handler+0x81/0xa3

Reported-by: Daniel Walter <sahne@xxxxxxx>
Tested-by: Richard Weinberger <richard@xxxxxx>
Tested-by: Toralf Förster <toralf.foerster@xxxxxx>
Tested-by: Daniel Walter <sahne@xxxxxxx>
Signed-off-by: Vivek Goyal <vgoyal@xxxxxxxxxx>
kernel/resource.c | 11 ++++-------
1 file changed, 4 insertions(+), 7 deletions(-)

Index: linux-2.6/kernel/resource.c
--- linux-2.6.orig/kernel/resource.c 2014-08-12 08:45:15.389969995 -0400
+++ linux-2.6/kernel/resource.c 2014-08-12 08:45:23.572009211 -0400
@@ -351,15 +351,12 @@ static int find_next_iomem_res(struct re
end = res->end;
BUG_ON(start >= end);

- read_lock(&resource_lock);
- if (first_level_children_only) {
- p = iomem_resource.child;
+ if (first_level_children_only)
sibling_only = true;
- } else
- p = &iomem_resource;

- while ((p = next_resource(p, sibling_only))) {
+ read_lock(&resource_lock);
+ for (p = iomem_resource.child; p; p = next_resource(p, sibling_only)) {
if (p->flags != res->flags)
if (name && strcmp(p->name, name))
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