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[PATCH 3/6] x86/mmap: properly account for stack randomization in mmap_base

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From: Rik van Riel <riel@redhat.com>

When RLIMIT_STACK is, for example, 256MB, the current code results in
a gap between the top of the task and mmap_base of 256MB, failing to
take into account the amount by which the stack address was randomized.
In other words, the stack gets less than RLIMIT_STACK space.

Ensure that the gap between the stack and mmap_base always takes stack
randomization into account.

From Daniel Micay's linux-hardened tree.

Reported-by: Florian Weimer <fweimer@redhat.com>
Signed-off-by: Daniel Micay <danielmicay@gmail.com>
Signed-off-by: Rik van Riel <riel@redhat.com>
---
 arch/x86/mm/mmap.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/arch/x86/mm/mmap.c b/arch/x86/mm/mmap.c
index 19ad095b41df..8c7ba1adb27b 100644
--- a/arch/x86/mm/mmap.c
+++ b/arch/x86/mm/mmap.c
@@ -95,13 +95,18 @@ unsigned long arch_mmap_rnd(void)
 static unsigned long mmap_base(unsigned long rnd, unsigned long task_size)
 {
 	unsigned long gap = rlimit(RLIMIT_STACK);
+	unsigned long pad = stack_maxrandom_size(task_size);
 	unsigned long gap_min, gap_max;
 
+	/* Values close to RLIM_INFINITY can overflow. */
+	if (gap + pad > gap)
+		gap += pad;
+
 	/*
 	 * Top of mmap area (just below the process stack).
 	 * Leave an at least ~128 MB hole with possible stack randomization.
 	 */
-	gap_min = SIZE_128M + stack_maxrandom_size(task_size);
+	gap_min = SIZE_128M;
 	gap_max = (task_size / 6) * 5;
 
 	if (gap < gap_min)
-- 
2.9.3

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[PATCH 0/6] move mmap_area and PIE binaries away from the stack riel@redhat.com - 2017-06-02 17:30 +0200
  [PATCH 1/6] binfmt_elf: document load_bias a little bit riel@redhat.com - 2017-06-02 17:30 +0200
    Re: [kernel-hardening] [PATCH 1/6] binfmt_elf: document load_bias a  little bit Kees Cook <keescook@chromium.org> - 2017-06-02 21:30 +0200
  [PATCH 6/6] powerpc,mmap: properly account for stack randomization in mmap_base riel@redhat.com - 2017-06-02 17:30 +0200
  [PATCH 4/6] arm64/mmap: properly account for stack randomization in mmap_base riel@redhat.com - 2017-06-02 17:30 +0200
  [PATCH 2/6] x86/elf: move 32 bit ELF_ET_DYN_BASE to 256MB riel@redhat.com - 2017-06-02 17:30 +0200
    Re: [PATCH 2/6] x86/elf: move 32 bit ELF_ET_DYN_BASE to 256MB Kees Cook <keescook@chromium.org> - 2017-06-03 06:30 +0200
      Re: [PATCH 2/6] x86/elf: move 32 bit ELF_ET_DYN_BASE to 256MB Daniel Micay <danielmicay@gmail.com> - 2017-06-03 14:00 +0200
      Re: [PATCH 2/6] x86/elf: move 32 bit ELF_ET_DYN_BASE to 256MB Rik van Riel <riel@redhat.com> - 2017-06-05 16:00 +0200
  [PATCH 3/6] x86/mmap: properly account for stack randomization in mmap_base riel@redhat.com - 2017-06-02 17:30 +0200
    Re: [kernel-hardening] [PATCH 3/6] x86/mmap: properly account for  stack randomization in mmap_base Kees Cook <keescook@chromium.org> - 2017-06-03 06:50 +0200
      Re: [kernel-hardening] [PATCH 3/6] x86/mmap: properly account for  stack randomization in mmap_base Daniel Micay <danielmicay@gmail.com> - 2017-06-03 14:20 +0200
  [PATCH 5/6] arm64: move COMPAT_ELF_ET_DYN_BASE lower in the address space riel@redhat.com - 2017-06-02 17:30 +0200
  Re: [kernel-hardening] [PATCH 0/6] move mmap_area and PIE binaries  away from the stack Kees Cook <keescook@chromium.org> - 2017-06-03 06:40 +0200
    Re: [kernel-hardening] [PATCH 0/6] move mmap_area and PIE binaries  away from the stack Daniel Micay <danielmicay@gmail.com> - 2017-06-03 14:20 +0200

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