Re: [PATCH] ALSA: isa: gus: Fix a possible null-pointer dereference in snd_gf1_mem_xfree()

[Jia-Ju Bai]

Thanks for the quick reply :)
I think you are right, and I did not consider "if (alloc->last == block)"
Sorry for the false report...


Best wishes,
Jia-Ju Bai

On 2019/7/23 21:47, Takashi Iwai wrote:

On Tue, 23 Jul 2019 15:40:20 +0200,
Jia-Ju Bai wrote:

In snd_gf1_mem_xfree(), there is an if statement on line 72 and line 74
to check whether block->next is NULL:
     if (block->next)

When block->next is NULL, block->next is used on line 84:
     block->next->prev = block->prev;

Thus, a possible null-pointer dereference may occur in this case.

There is already a check beforehand:

        if (alloc->last == block) {

and the code path you're referring to is only after this check fails,
i.e. it's no last entry, hence block->next can be never NULL.

So the current code is OK.


thanks,

Takashi

To fix this possible bug, block->next is checked before using it.

This bug is found by a static analysis tool STCheck written by us.

Signed-off-by: Jia-Ju Bai <baijiaju1...@gmail.com>
---
  sound/isa/gus/gus_mem.c | 3 ++-
  1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/sound/isa/gus/gus_mem.c b/sound/isa/gus/gus_mem.c
index cb02d18dde60..ed6205b88057 100644
--- a/sound/isa/gus/gus_mem.c
+++ b/sound/isa/gus/gus_mem.c
@@ -81,7 +81,8 @@ int snd_gf1_mem_xfree(struct snd_gf1_mem * alloc, struct 
snd_gf1_mem_block * blo
                if (block->prev)
                        block->prev->next = NULL;
        } else {
-               block->next->prev = block->prev;
+               if (block->next)
+                       block->next->prev = block->prev;
                if (block->prev)
                        block->prev->next = block->next;
        }
--
2.17.0