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41 lines
1.8 KiB
Diff
41 lines
1.8 KiB
Diff
From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
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From: Daniel Micay <danielmicay@gmail.com>
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Date: Sun, 12 Mar 2017 17:49:13 -0400
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Subject: [PATCH] on 64-bit, zero the leading stack canary byte
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This reduces entropy of the canary from 64-bit to 56-bit in exchange for
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mitigating non-terminated C string overflows.
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---
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libc/bionic/__libc_init_main_thread.cpp | 11 +++++++++--
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1 file changed, 9 insertions(+), 2 deletions(-)
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diff --git a/libc/bionic/__libc_init_main_thread.cpp b/libc/bionic/__libc_init_main_thread.cpp
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index 2e3967daa..f5128de9e 100644
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--- a/libc/bionic/__libc_init_main_thread.cpp
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+++ b/libc/bionic/__libc_init_main_thread.cpp
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@@ -49,8 +49,11 @@ uintptr_t __stack_chk_guard[PAGE_SIZE / sizeof(uintptr_t)] = {0};
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static pthread_internal_t main_thread;
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-void __libc_init_global_stack_chk_guard(KernelArgumentBlock& args) {
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-}
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+#if __LP64__
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+static const uintptr_t canary_mask = __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__ ?
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+ 0xffffffffffffff00UL :
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+ 0x00ffffffffffffffUL;
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+#endif
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// Setup for the main thread. For dynamic executables, this is called by the
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// linker _before_ libc is mapped in memory. This means that all writes to
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@@ -108,6 +111,10 @@ extern "C" void __libc_init_main_thread_late() {
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// before we initialize the TLS. Dynamic executables will initialize their copy of the global
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// stack protector from the one in the main thread's TLS.
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__libc_safe_arc4random_buf(&__stack_chk_guard[0], sizeof(__stack_chk_guard[0]));
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+#if __LP64__
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+ // Sacrifice 8 bits of entropy on 64-bit to mitigate non-terminated C string overflows
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+ __stack_chk_guard[0] &= canary_mask;
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+#endif
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if (mprotect(__stack_chk_guard, sizeof(__stack_chk_guard), PROT_READ) == -1) {
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async_safe_fatal("mprotect __stack_chk_guard: %s", strerror(errno));
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}
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