Re: [kernel-hardening] Re: [PATCH 9/9] mm: SLUB hardened usercopy support
From: Kees Cook <hidden>
Date: 2016-07-07 18:57:21
Also in:
linux-mm, linuxppc-dev, lkml, sparclinux
On Thu, Jul 7, 2016 at 12:35 AM, Michael Ellerman [off-list ref] wrote:
Kees Cook [off-list ref] writes:quoted
Under CONFIG_HARDENED_USERCOPY, this adds object size checking to the SLUB allocator to catch any copies that may span objects. Based on code from PaX and grsecurity. Signed-off-by: Kees Cook <redacted>quoted
diff --git a/mm/slub.c b/mm/slub.c index 825ff4505336..0c8ace04f075 100644 --- a/mm/slub.c +++ b/mm/slub.c@@ -3614,6 +3614,33 @@ void *__kmalloc_node(size_t size, gfp_t flags, int node) EXPORT_SYMBOL(__kmalloc_node); #endif +#ifdef CONFIG_HARDENED_USERCOPY +/* + * Rejects objects that are incorrectly sized. + * + * Returns NULL if check passes, otherwise const char * to name of cache + * to indicate an error. + */ +const char *__check_heap_object(const void *ptr, unsigned long n, + struct page *page) +{ + struct kmem_cache *s; + unsigned long offset; + + /* Find object. */ + s = page->slab_cache; + + /* Find offset within object. */ + offset = (ptr - page_address(page)) % s->size; + + /* Allow address range falling entirely within object size. */ + if (offset <= s->object_size && n <= s->object_size - offset) + return NULL; + + return s->name; +}I gave this a quick spin on powerpc, it blew up immediately :)
Wheee :) This series is rather easy to test: blows up REALLY quickly if it's wrong. ;) FWIW, -next also has a bunch of additional lkdtm tests for the various protections and directions.
quoted hunk
Brought up 16 CPUs usercopy: kernel memory overwrite attempt detected to c0000001fe023868 (kmalloc-16) (9 bytes) CPU: 8 PID: 103 Comm: kdevtmpfs Not tainted 4.7.0-rc3-00098-g09d9556ae5d1 #55 Call Trace: [c0000001fa0cfb40] [c0000000009bdbe8] dump_stack+0xb0/0xf0 (unreliable) [c0000001fa0cfb80] [c00000000029cf44] __check_object_size+0x74/0x320 [c0000001fa0cfc00] [c00000000005d4d0] copy_from_user+0x60/0xd4 [c0000001fa0cfc40] [c00000000022b6cc] memdup_user+0x5c/0xf0 [c0000001fa0cfc80] [c00000000022b90c] strndup_user+0x7c/0x110 [c0000001fa0cfcc0] [c0000000002d6c28] SyS_mount+0x58/0x180 [c0000001fa0cfd10] [c0000000005ee908] devtmpfsd+0x98/0x210 [c0000001fa0cfd80] [c0000000000df810] kthread+0x110/0x130 [c0000001fa0cfe30] [c0000000000095e8] ret_from_kernel_thread+0x5c/0x74 SLUB tracing says: TRACE kmalloc-16 alloc 0xc0000001fe023868 inuse=186 fp=0x (null) Which is not 16-byte aligned, which seems to be caused by the red zone? The following patch fixes it for me, but I don't know SLUB enough to say if it's always correct.diff --git a/mm/slub.c b/mm/slub.c index 0c8ace04f075..66191ea4545a 100644 --- a/mm/slub.c +++ b/mm/slub.c@@ -3630,6 +3630,9 @@ const char *__check_heap_object(const void *ptr, unsigned long n, /* Find object. */ s = page->slab_cache; + /* Subtract red zone if enabled */ + ptr = restore_red_left(s, ptr); +
Ah, interesting. Just to make sure: you've built with CONFIG_SLUB_DEBUG and either CONFIG_SLUB_DEBUG_ON or booted with either slub_debug or slub_debug=z ? Thanks for the slub fix! I wonder if this code should be using size_from_object() instead of s->size? (It looks like slab is already handling this via the obj_offset() call.) -Kees
/* Find offset within object. */
offset = (ptr - page_address(page)) % s->size;
cheers-- Kees Cook Chrome OS & Brillo Security -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>