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SubjectRe: kernel BUG at slab.c:1542!(2.4.1-pre9)
Daniel Phillips wrote:
>
> I don't know much about the history of this bug but it's quite clear
> it's deliberately inserted:
>
> void * kmalloc (size_t size, int flags)
> <if allocation succeeds, exit>
> BUG(); // too big size
> return NULL;
>

I sent the below patch to Linus earlier today. I didn't copy
any mailing list because it's a bit security-related. Oh well.

If wmem_max is set to > 256kbytes, an application can
set SO_SNDBUF on a unix domain stream socket to >256k
and do a write() of >256k. unix_stream_sendmsg()
then tries to kmalloc >128k and the kernel dies.
I don't like the idea of simply returning NULL from kmalloc in
this case, because an application which does

setsockopt(fd, SOL_SOCKET, SO_SNDBUF, 256kbytes);
write(fd, buf, 256kbytes);

will work perfectly well, until someone increases wmem_max
to greater than 256k. This administrative action will
cause the above application to mysteriously start failing.




--- linux-2.4.1-pre10/include/linux/slab.h Mon Jan 1 18:29:35 2001
+++ linux-akpm/include/linux/slab.h Wed Jan 24 12:29:34 2001
@@ -75,6 +75,9 @@
extern kmem_cache_t *fs_cachep;
extern kmem_cache_t *sigact_cachep;

+/* Largest kmalloc which we support */
+extern size_t kmalloc_max;
+
#endif /* __KERNEL__ */

#endif /* _LINUX_SLAB_H */
--- linux-2.4.1-pre10/mm/slab.c Tue Jan 23 19:28:16 2001
+++ linux-akpm/mm/slab.c Wed Jan 24 12:57:47 2001
@@ -360,6 +360,9 @@
/* Place maintainer for reaping. */
static kmem_cache_t *clock_searchp = &cache_cache;

+/* Largest kmalloc which we support */
+size_t kmalloc_max;
+
#define cache_chain (cache_cache.next)

#ifdef CONFIG_SMP
@@ -455,6 +458,7 @@
SLAB_CACHE_DMA|SLAB_HWCACHE_ALIGN, NULL, NULL);
if (!sizes->cs_dmacachep)
BUG();
+ kmalloc_max = sizes->cs_size;
sizes++;
} while (sizes->cs_size);
}
--- linux-2.4.1-pre10/kernel/ksyms.c Tue Jan 23 19:28:16 2001
+++ linux-akpm/kernel/ksyms.c Wed Jan 24 12:17:28 2001
@@ -104,6 +104,7 @@
EXPORT_SYMBOL(kmem_cache_alloc);
EXPORT_SYMBOL(kmem_cache_free);
EXPORT_SYMBOL(kmalloc);
+EXPORT_SYMBOL(kmalloc_max);
EXPORT_SYMBOL(kfree);
EXPORT_SYMBOL(vfree);
EXPORT_SYMBOL(__vmalloc);
--- linux-2.4.1-pre10/net/unix/af_unix.c Tue Jan 23 19:28:16 2001
+++ linux-akpm/net/unix/af_unix.c Wed Jan 24 12:57:35 2001
@@ -1319,6 +1319,10 @@
if (size > sk->sndbuf/2 - 16)
size = sk->sndbuf/2 - 16;

+ /* Avoid oversized kmallocs */
+ if (size > kmalloc_max / 2 - 16)
+ size = kmalloc_max / 2 - 16;
+
/*
* Keep to page sized kmalloc()'s as various people
* have suggested. Big mallocs stress the vm too
-
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