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From:   Alexandre Ghiti <alex@ghiti.fr>
To:     Andrew Morton <akpm@linux-foundation.org>
Cc:     Paul Walmsley <paul.walmsley@sifive.com>,
        Luis Chamberlain <mcgrof@kernel.org>,
        Christoph Hellwig <hch@lst.de>,
        Russell King <linux@armlinux.org.uk>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will.deacon@arm.com>,
        Ralf Baechle <ralf@linux-mips.org>,
        Paul Burton <paul.burton@mips.com>,
        James Hogan <jhogan@kernel.org>,
        Palmer Dabbelt <palmer@sifive.com>,
        Albert Ou <aou@eecs.berkeley.edu>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Kees Cook <keescook@chromium.org>,
        linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
        linux-mips@vger.kernel.org, linux-riscv@lists.infradead.org,
        linux-fsdevel@vger.kernel.org, linux-mm@kvack.org,
        Alexandre Ghiti <alex@ghiti.fr>
Subject: [PATCH v6 06/14] arm: Properly account for stack randomization and stack guard gap
Date:   Thu,  8 Aug 2019 02:17:48 -0400
Message-Id: <20190808061756.19712-7-alex@ghiti.fr>
In-Reply-To: <20190808061756.19712-1-alex@ghiti.fr>
References: <20190808061756.19712-1-alex@ghiti.fr>
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This commit takes care of stack randomization and stack guard gap when
computing mmap base address and checks if the task asked for randomization.

This fixes the problem uncovered and not fixed for arm here:
https://lkml.kernel.org/r/20170622200033.25714-1-riel@redhat.com

Signed-off-by: Alexandre Ghiti <alex@ghiti.fr>
Acked-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Luis Chamberlain <mcgrof@kernel.org>
---
 arch/arm/mm/mmap.c | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/arch/arm/mm/mmap.c b/arch/arm/mm/mmap.c
index f866870db749..bff3d00bda5b 100644
--- a/arch/arm/mm/mmap.c
+++ b/arch/arm/mm/mmap.c
@@ -18,8 +18,9 @@
 	 (((pgoff)<<PAGE_SHIFT) & (SHMLBA-1)))
 
 /* gap between mmap and stack */
-#define MIN_GAP (128*1024*1024UL)
-#define MAX_GAP ((TASK_SIZE)/6*5)
+#define MIN_GAP		(128*1024*1024UL)
+#define MAX_GAP		((TASK_SIZE)/6*5)
+#define STACK_RND_MASK	(0x7ff >> (PAGE_SHIFT - 12))
 
 static int mmap_is_legacy(struct rlimit *rlim_stack)
 {
@@ -35,6 +36,15 @@ static int mmap_is_legacy(struct rlimit *rlim_stack)
 static unsigned long mmap_base(unsigned long rnd, struct rlimit *rlim_stack)
 {
 	unsigned long gap = rlim_stack->rlim_cur;
+	unsigned long pad = stack_guard_gap;
+
+	/* Account for stack randomization if necessary */
+	if (current->flags & PF_RANDOMIZE)
+		pad += (STACK_RND_MASK << PAGE_SHIFT);
+
+	/* Values close to RLIM_INFINITY can overflow. */
+	if (gap + pad > gap)
+		gap += pad;
 
 	if (gap < MIN_GAP)
 		gap = MIN_GAP;
-- 
2.20.1