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Message-Id: <20200327064820.12602-5-keescook@chromium.org>
Date: Thu, 26 Mar 2020 23:48:18 -0700
From: Kees Cook <keescook@...omium.org>
To: Borislav Petkov <bp@...en8.de>
Cc: Kees Cook <keescook@...omium.org>,
Jason Gunthorpe <jgg@...lanox.com>,
Catalin Marinas <catalin.marinas@....com>,
Hector Marco-Gisbert <hecmargi@....es>,
Jason Gunthorpe <jgg@...pe.ca>,
Russell King <linux@...linux.org.uk>,
Will Deacon <will@...nel.org>,
Jann Horn <jannh@...gle.com>,
x86@...nel.org,
linux-arm-kernel@...ts.infradead.org,
kernel-hardening@...ts.openwall.com,
linux-kernel@...r.kernel.org
Subject: [PATCH v5 4/6] arm32/64, elf: Add tables to document READ_IMPLIES_EXEC
Add tables to document the current behavior of READ_IMPLIES_EXEC in
preparation for changing the behavior for both arm64 and arm.
Signed-off-by: Kees Cook <keescook@...omium.org>
Reviewed-by: Jason Gunthorpe <jgg@...lanox.com>
Reviewed-by: Catalin Marinas <catalin.marinas@....com>
---
arch/arm/kernel/elf.c | 24 +++++++++++++++++++++---
arch/arm64/include/asm/elf.h | 20 ++++++++++++++++++++
2 files changed, 41 insertions(+), 3 deletions(-)
diff --git a/arch/arm/kernel/elf.c b/arch/arm/kernel/elf.c
index 182422981386..5ccd4aced6cc 100644
--- a/arch/arm/kernel/elf.c
+++ b/arch/arm/kernel/elf.c
@@ -78,9 +78,27 @@ void elf_set_personality(const struct elf32_hdr *x)
EXPORT_SYMBOL(elf_set_personality);
/*
- * Set READ_IMPLIES_EXEC if:
- * - the binary requires an executable stack
- * - we're running on a CPU which doesn't support NX.
+ * An executable for which elf_read_implies_exec() returns TRUE will
+ * have the READ_IMPLIES_EXEC personality flag set automatically.
+ *
+ * The decision process for determining the results are:
+ *
+ * CPU: | lacks NX* | has NX |
+ * ELF: | | |
+ * ---------------------|------------|------------|
+ * missing PT_GNU_STACK | exec-all | exec-all |
+ * PT_GNU_STACK == RWX | exec-all | exec-all |
+ * PT_GNU_STACK == RW | exec-all | exec-none |
+ *
+ * exec-all : all PROT_READ user mappings are executable, except when
+ * backed by files on a noexec-filesystem.
+ * exec-none : only PROT_EXEC user mappings are executable.
+ *
+ * *this column has no architectural effect: NX markings are ignored by
+ * hardware, but may have behavioral effects when "wants X" collides with
+ * "cannot be X" constraints in memory permission flags, as in
+ * https://lkml.kernel.org/r/20190418055759.GA3155@mellanox.com
+ *
*/
int arm_elf_read_implies_exec(int executable_stack)
{
diff --git a/arch/arm64/include/asm/elf.h b/arch/arm64/include/asm/elf.h
index b618017205a3..986ecf41fc0f 100644
--- a/arch/arm64/include/asm/elf.h
+++ b/arch/arm64/include/asm/elf.h
@@ -96,6 +96,26 @@
*/
#define elf_check_arch(x) ((x)->e_machine == EM_AARCH64)
+/*
+ * An executable for which elf_read_implies_exec() returns TRUE will
+ * have the READ_IMPLIES_EXEC personality flag set automatically.
+ *
+ * The decision process for determining the results are:
+ *
+ * CPU*: | arm32 | arm64 |
+ * ELF: | | |
+ * ---------------------|------------|------------|
+ * missing PT_GNU_STACK | exec-all | exec-all |
+ * PT_GNU_STACK == RWX | exec-all | exec-all |
+ * PT_GNU_STACK == RW | exec-none | exec-none |
+ *
+ * exec-all : all PROT_READ user mappings are executable, except when
+ * backed by files on a noexec-filesystem.
+ * exec-none : only PROT_EXEC user mappings are executable.
+ *
+ * *all arm64 CPUs support NX, so there is no "lacks NX" column.
+ *
+ */
#define elf_read_implies_exec(ex,stk) (stk != EXSTACK_DISABLE_X)
#define CORE_DUMP_USE_REGSET
--
2.20.1
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