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Subject[PATCH] binfmt_elf: Update READ_IMPLIES_EXEC logic for modern CPUs
The READ_IMPLIES_EXEC work-around was designed for old CPUs lacking NX
(to have the visible permission flags on memory regions reflect reality:
they are all executable), and for old toolchains that lacked the ELF
PT_GNU_STACK marking (under the assumption than toolchains that couldn't
even specify memory protection flags may have it wrong for all memory
regions).

This logic is sensible, but was implemented in a way that equated having
a PT_GNU_STACK marked executable as being as "broken" as lacking the
PT_GNU_STACK marking entirely. This is not a reasonable assumption
for CPUs that have had NX support from the start (or very close to
the start). This confusion has led to situations where modern 64-bit
programs with explicitly marked executable stack are forced into the
READ_IMPLIES_EXEC state when no such thing is needed. (And leads to
unexpected failures when mmap()ing regions of device driver memory that
wish to disallow VM_EXEC[1].)

To fix this, elf_read_implies_exec() is adjusted on arm64 (where NX has
always existed and all toolchains include PT_GNU_STACK), and x86 is
adjusted to handle this combination of possible outcomes:

CPU: | lacks NX | has NX, ia32 | has NX, x86_64 |
ELF: | | | |
------------------------------|------------------|------------------|
missing GNU_STACK | needs RIE | needs RIE | no RIE |
GNU_STACK == RWX | needs RIE | no RIE: stack X | no RIE: stack X |
GNU_STACK == RW | needs RIE | no RIE: stack NX | no RIE: stack NX |

This has the effect of making binfmt_elf's EXSTACK_DEFAULT actually take
on the correct architecture default of being non-executable on arm64 and
x86_64, and being executable on ia32.

[1] https://lkml.kernel.org/r/20190418055759.GA3155@mellanox.com

Suggested-by: Hector Marco-Gisbert <hecmargi@upv.es>
Signed-off-by: Kees Cook <keescook@chromium.org>
---
arch/arm64/include/asm/elf.h | 9 ++++++++-
arch/x86/include/asm/elf.h | 24 +++++++++++++++++++++---
2 files changed, 29 insertions(+), 4 deletions(-)

diff --git a/arch/arm64/include/asm/elf.h b/arch/arm64/include/asm/elf.h
index 6adc1a90e7e6..7fbd295a76d2 100644
--- a/arch/arm64/include/asm/elf.h
+++ b/arch/arm64/include/asm/elf.h
@@ -107,7 +107,14 @@
*/
#define elf_check_arch(x) ((x)->e_machine == EM_AARCH64)

-#define elf_read_implies_exec(ex,stk) (stk != EXSTACK_DISABLE_X)
+/*
+ * 64-bit processes should not automatically gain READ_IMPLIES_EXEC. Only
+ * 32-bit processes without PT_GNU_STACK should trigger READ_IMPLIES_EXEC
+ * out of an abundance of caution against ancient toolchains not knowing
+ * how to mark memory protection flags correctly.
+ */
+#define elf_read_implies_exec(ex, stk) \
+ (is_compat_task() && stk == EXSTACK_DEFAULT)

#define CORE_DUMP_USE_REGSET
#define ELF_EXEC_PAGESIZE PAGE_SIZE
diff --git a/arch/x86/include/asm/elf.h b/arch/x86/include/asm/elf.h
index 69c0f892e310..87d9cf7643b4 100644
--- a/arch/x86/include/asm/elf.h
+++ b/arch/x86/include/asm/elf.h
@@ -280,10 +280,28 @@ extern u32 elf_hwcap2;

/*
* An executable for which elf_read_implies_exec() returns TRUE will
- * have the READ_IMPLIES_EXEC personality flag set automatically.
+ * have the READ_IMPLIES_EXEC personality flag set automatically. This
+ * is needed either to show the truth about a memory mapping (i.e. CPUs
+ * that lack NX have all memory implicitly executable, so this makes
+ * sure that the visible permissions reflect reality), or to deal with
+ * old toolchains on new CPUs. Old binaries entirely lacking a GNU_STACK
+ * indicate they were likely built with a toolchain that has no idea about
+ * memory permissions, and so we must default to the lowest reasonable
+ * common denominator for the architecture: on ia32 we assume all memory
+ * to be executable by default, and on x86_64 we assume all memory to be
+ * non-executable by default.
+ *
+ * CPU: | lacks NX | has NX, ia32 | has NX, x86_64 |
+ * ELF: | | | |
+ * ------------------------------|------------------|------------------|
+ * missing GNU_STACK | needs RIE | needs RIE | no RIE |
+ * GNU_STACK == RWX | needs RIE | no RIE: stack X | no RIE: stack X |
+ * GNU_STACK == RW | needs RIE | no RIE: stack NX | no RIE: stack NX |
+ *
*/
-#define elf_read_implies_exec(ex, executable_stack) \
- (executable_stack != EXSTACK_DISABLE_X)
+#define elf_read_implies_exec(ex, stk) \
+ (!(__supported_pte_mask & _PAGE_NX) ? 1 : \
+ (mmap_is_ia32() && stk == EXSTACK_DEFAULT))

struct task_struct;

--
2.17.1

--
Kees Cook

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