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authorAlexander Popov <alex.popov@linux.com>2018-08-17 01:16:58 +0300
committerKees Cook <keescook@chromium.org>2018-09-04 10:35:47 -0700
commitafaef01c001537fa97a25092d7f54d764dc7d8c1 (patch)
tree199a05427ea4c1e0c735058f322a5b21625b9ecd /kernel/stackleak.c
parent57361846b52bc686112da6ca5368d11210796804 (diff)
downloadlinux-afaef01c001537fa97a25092d7f54d764dc7d8c1.tar.bz2
x86/entry: Add STACKLEAK erasing the kernel stack at the end of syscalls
The STACKLEAK feature (initially developed by PaX Team) has the following benefits: 1. Reduces the information that can be revealed through kernel stack leak bugs. The idea of erasing the thread stack at the end of syscalls is similar to CONFIG_PAGE_POISONING and memzero_explicit() in kernel crypto, which all comply with FDP_RIP.2 (Full Residual Information Protection) of the Common Criteria standard. 2. Blocks some uninitialized stack variable attacks (e.g. CVE-2017-17712, CVE-2010-2963). That kind of bugs should be killed by improving C compilers in future, which might take a long time. This commit introduces the code filling the used part of the kernel stack with a poison value before returning to userspace. Full STACKLEAK feature also contains the gcc plugin which comes in a separate commit. The STACKLEAK feature is ported from grsecurity/PaX. More information at: https://grsecurity.net/ https://pax.grsecurity.net/ This code is modified from Brad Spengler/PaX Team's code in the last public patch of grsecurity/PaX based on our understanding of the code. Changes or omissions from the original code are ours and don't reflect the original grsecurity/PaX code. Performance impact: Hardware: Intel Core i7-4770, 16 GB RAM Test #1: building the Linux kernel on a single core 0.91% slowdown Test #2: hackbench -s 4096 -l 2000 -g 15 -f 25 -P 4.2% slowdown So the STACKLEAK description in Kconfig includes: "The tradeoff is the performance impact: on a single CPU system kernel compilation sees a 1% slowdown, other systems and workloads may vary and you are advised to test this feature on your expected workload before deploying it". Signed-off-by: Alexander Popov <alex.popov@linux.com> Acked-by: Thomas Gleixner <tglx@linutronix.de> Reviewed-by: Dave Hansen <dave.hansen@linux.intel.com> Acked-by: Ingo Molnar <mingo@kernel.org> Signed-off-by: Kees Cook <keescook@chromium.org>
Diffstat (limited to 'kernel/stackleak.c')
-rw-r--r--kernel/stackleak.c62
1 files changed, 62 insertions, 0 deletions
diff --git a/kernel/stackleak.c b/kernel/stackleak.c
new file mode 100644
index 000000000000..deba0d8992f9
--- /dev/null
+++ b/kernel/stackleak.c
@@ -0,0 +1,62 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * This code fills the used part of the kernel stack with a poison value
+ * before returning to userspace. It's part of the STACKLEAK feature
+ * ported from grsecurity/PaX.
+ *
+ * Author: Alexander Popov <alex.popov@linux.com>
+ *
+ * STACKLEAK reduces the information which kernel stack leak bugs can
+ * reveal and blocks some uninitialized stack variable attacks.
+ */
+
+#include <linux/stackleak.h>
+
+asmlinkage void stackleak_erase(void)
+{
+ /* It would be nice not to have 'kstack_ptr' and 'boundary' on stack */
+ unsigned long kstack_ptr = current->lowest_stack;
+ unsigned long boundary = (unsigned long)end_of_stack(current);
+ unsigned int poison_count = 0;
+ const unsigned int depth = STACKLEAK_SEARCH_DEPTH / sizeof(unsigned long);
+
+ /* Check that 'lowest_stack' value is sane */
+ if (unlikely(kstack_ptr - boundary >= THREAD_SIZE))
+ kstack_ptr = boundary;
+
+ /* Search for the poison value in the kernel stack */
+ while (kstack_ptr > boundary && poison_count <= depth) {
+ if (*(unsigned long *)kstack_ptr == STACKLEAK_POISON)
+ poison_count++;
+ else
+ poison_count = 0;
+
+ kstack_ptr -= sizeof(unsigned long);
+ }
+
+ /*
+ * One 'long int' at the bottom of the thread stack is reserved and
+ * should not be poisoned (see CONFIG_SCHED_STACK_END_CHECK=y).
+ */
+ if (kstack_ptr == boundary)
+ kstack_ptr += sizeof(unsigned long);
+
+ /*
+ * Now write the poison value to the kernel stack. Start from
+ * 'kstack_ptr' and move up till the new 'boundary'. We assume that
+ * the stack pointer doesn't change when we write poison.
+ */
+ if (on_thread_stack())
+ boundary = current_stack_pointer;
+ else
+ boundary = current_top_of_stack();
+
+ while (kstack_ptr < boundary) {
+ *(unsigned long *)kstack_ptr = STACKLEAK_POISON;
+ kstack_ptr += sizeof(unsigned long);
+ }
+
+ /* Reset the 'lowest_stack' value for the next syscall */
+ current->lowest_stack = current_top_of_stack() - THREAD_SIZE/64;
+}
+