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From: Kees Cook <>
To: Borislav Petkov <>
Cc: Kees Cook <>,
	Hector Marco-Gisbert <>,
	Jason Gunthorpe <>,
	Jason Gunthorpe <>,
	Catalin Marinas <>,
	Russell King <>,
	Will Deacon <>, Jann Horn <>,,,,
Subject: [PATCH v5 2/6] x86/elf: Split READ_IMPLIES_EXEC from executable PT_GNU_STACK
Date: Thu, 26 Mar 2020 23:48:16 -0700
Message-ID: <> (raw)
In-Reply-To: <>

The READ_IMPLIES_EXEC work-around was designed for old toolchains that
lacked the ELF PT_GNU_STACK marking under the assumption that toolchains
that couldn't specify executable permission flags for the stack may not
know how to do it correctly for any memory region.

This logic is sensible for having ancient binaries coexist in a system
with possibly NX memory, 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. Things like unmarked assembly and stack
trampolines may cause PT_GNU_STACK to need an executable bit, but they
do not imply all mappings must be executable.

This confusion has led to situations where modern 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].)

In looking for other reasons for the READ_IMPLIES_EXEC behavior, Jann
Horn noted that glibc thread stacks have always been marked RWX (until
2003 when they started tracking the PT_GNU_STACK flag instead[2]). And
musl doesn't support executable stacks at all[3]. As such, no breakage
for multithreaded applications is expected from this change.


Suggested-by: Hector Marco-Gisbert <>
Signed-off-by: Kees Cook <>
Reviewed-by: Jason Gunthorpe <>
 arch/x86/include/asm/elf.h | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/asm/elf.h b/arch/x86/include/asm/elf.h
index ee459d4c3b45..397a1c74433e 100644
--- a/arch/x86/include/asm/elf.h
+++ b/arch/x86/include/asm/elf.h
@@ -288,12 +288,13 @@ extern u32 elf_hwcap2;
  * ELF:                 |            |                  |                |
  * ---------------------|------------|------------------|----------------|
  * missing PT_GNU_STACK | exec-all   | exec-all         | exec-all       |
- * PT_GNU_STACK == RWX  | exec-all   | exec-all         | exec-all       |
+ * PT_GNU_STACK == RWX  | exec-stack | exec-stack       | exec-stack     |
  * PT_GNU_STACK == RW   | exec-none  | 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.
+ *  exec-stack: only the stack and 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
@@ -302,7 +303,7 @@ extern u32 elf_hwcap2;
 #define elf_read_implies_exec(ex, executable_stack)	\
-	(executable_stack != EXSTACK_DISABLE_X)
+	(executable_stack == EXSTACK_DEFAULT)
 struct task_struct;

  parent reply index

Thread overview: 7+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-03-27  6:48 [PATCH v5 0/6] binfmt_elf: Update READ_IMPLIES_EXEC logic for modern CPUs Kees Cook
2020-03-27  6:48 ` [PATCH v5 1/6] x86/elf: Add table to document READ_IMPLIES_EXEC Kees Cook
2020-03-27  6:48 ` Kees Cook [this message]
2020-03-27  6:48 ` [PATCH v5 3/6] x86/elf: Disable automatic READ_IMPLIES_EXEC for 64-bit address spaces Kees Cook
2020-03-27  6:48 ` [PATCH v5 4/6] arm32/64, elf: Add tables to document READ_IMPLIES_EXEC Kees Cook
2020-03-27  6:48 ` [PATCH v5 5/6] arm32/64, elf: Split READ_IMPLIES_EXEC from executable PT_GNU_STACK Kees Cook
2020-03-27  6:48 ` [PATCH v5 6/6] arm64, elf: Disable automatic READ_IMPLIES_EXEC for 64-bit address spaces Kees Cook

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