Commit 920d451f authored Feb 25, 2016 by Laura Abbott Committed by Kees Cook Mar 01, 2016

lkdtm: Add read/write after free tests for buddy memory



The current tests for read/write after free work on slab
allocated memory. Memory straight from the buddy allocator
may behave slightly differently and have a different set
of parameters to test. Add tests for those cases as well.

On a basic x86 boot:

 # echo WRITE_BUDDY_AFTER_FREE > /sys/kernel/debug/provoke-crash/DIRECT
[   22.291950] lkdtm: Performing direct entry WRITE_BUDDY_AFTER_FREE
[   22.292983] lkdtm: Writing to the buddy page before free
[   22.293950] lkdtm: Attempting bad write to the buddy page after free

 # echo READ_BUDDY_AFTER_FREE > /sys/kernel/debug/provoke-crash/DIRECT
[   32.375601] lkdtm: Performing direct entry READ_BUDDY_AFTER_FREE
[   32.379896] lkdtm: Value in memory before free: 12345678
[   32.383854] lkdtm: Attempting to read from freed memory
[   32.389309] lkdtm: Buddy page was not poisoned

On x86 with CONFIG_DEBUG_PAGEALLOC and debug_pagealloc=on:

 # echo WRITE_BUDDY_AFTER_FREE > /sys/kernel/debug/provoke-crash/DIRECT
[   17.475533] lkdtm: Performing direct entry WRITE_BUDDY_AFTER_FREE
[   17.477360] lkdtm: Writing to the buddy page before free
[   17.479089] lkdtm: Attempting bad write to the buddy page after free
[   17.480904] BUG: unable to handle kernel paging request at
ffff88000ebd8000

 # echo READ_BUDDY_AFTER_FREE > /sys/kernel/debug/provoke-crash/DIRECT
[   14.606433] lkdtm: Performing direct entry READ_BUDDY_AFTER_FREE
[   14.607447] lkdtm: Value in memory before free: 12345678
[   14.608161] lkdtm: Attempting to read from freed memory
[   14.608860] BUG: unable to handle kernel paging request at
ffff88000eba3000

Note that arches without ARCH_SUPPORTS_DEBUG_PAGEALLOC may not
produce the same crash.

Signed-off-by: Laura Abbott <labbott@fedoraproject.org>
Signed-off-by: Kees Cook <keescook@chromium.org>

parent 250a8988

drivers/misc/lkdtm.c

+45 −0

Original line number	Diff line number	Diff line
		@@ -93,6 +93,8 @@ enum ctype {
		CT_OVERWRITE_ALLOCATION,
		CT_WRITE_AFTER_FREE,
		CT_READ_AFTER_FREE,
		CT_WRITE_BUDDY_AFTER_FREE,
		CT_READ_BUDDY_AFTER_FREE,
		CT_SOFTLOCKUP,
		CT_HARDLOCKUP,
		CT_SPINLOCKUP,
		@@ -131,6 +133,8 @@ static char* cp_type[] = {
		"OVERWRITE_ALLOCATION",
		"WRITE_AFTER_FREE",
		"READ_AFTER_FREE",
		"WRITE_BUDDY_AFTER_FREE",
		"READ_BUDDY_AFTER_FREE",
		"SOFTLOCKUP",
		"HARDLOCKUP",
		"SPINLOCKUP",
		@@ -464,6 +468,47 @@ static void lkdtm_do_action(enum ctype which)
		kfree(val);
		break;
		}
		case CT_WRITE_BUDDY_AFTER_FREE: {
		unsigned long p = __get_free_page(GFP_KERNEL);
		if (!p)
		break;
		pr_info("Writing to the buddy page before free\n");
		memset((void *)p, 0x3, PAGE_SIZE);
		free_page(p);
		schedule();
		pr_info("Attempting bad write to the buddy page after free\n");
		memset((void *)p, 0x78, PAGE_SIZE);
		break;
		}
		case CT_READ_BUDDY_AFTER_FREE: {
		unsigned long p = __get_free_page(GFP_KERNEL);
		int saw, *val = kmalloc(1024, GFP_KERNEL);
		int *base;

		if (!p)
		break;

		if (!val)
		break;

		base = (int *)p;

		*val = 0x12345678;
		base[0] = *val;
		pr_info("Value in memory before free: %x\n", base[0]);
		free_page(p);
		pr_info("Attempting to read from freed memory\n");
		saw = base[0];
		if (saw != *val) {
		/* Good! Poisoning happened, so declare a win. */
		pr_info("Buddy page correctly poisoned, calling BUG\n");
		BUG();
		}
		pr_info("Buddy page was not poisoned\n");

		kfree(val);
		break;
		}
		case CT_SOFTLOCKUP:
		preempt_disable();
		for (;;)

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