[PATCH v2] binfmt_elf: Update READ_IMPLIES_EXEC logic for modern CPUs

[Kees Cook]

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 that 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 toolchains have implemented PT_GNU_STACK for a while),
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@xxxxxxxxxxxx

Suggested-by: Hector Marco-Gisbert <hecmargi@xxxxxx>
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
v2: adjust arm64 to avoid is_compat_task() (marc.w.gonzalez)
---
 arch/arm64/include/asm/elf.h |  8 +++++++-
 arch/x86/include/asm/elf.h   | 24 +++++++++++++++++++++---
 2 files changed, 28 insertions(+), 4 deletions(-)

diff --git a/arch/arm64/include/asm/elf.h b/arch/arm64/include/asm/elf.h
index 6adc1a90e7e6..f1bb4b388b8f 100644
--- a/arch/arm64/include/asm/elf.h
+++ b/arch/arm64/include/asm/elf.h
@@ -107,7 +107,13 @@
  */
 #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 compat_elf_read_implies_exec(ex, stk)	(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..5e65f1dcefc9 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) ||			\
+	 (mmap_is_ia32() && stk == EXSTACK_DEFAULT))
 
 struct task_struct;
 
-- 
2.17.1


-- 
Kees Cook