Merge tag 'core_core_updates_for_5.8' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull READ_IMPLIES_EXEC changes from Borislav Petkov:
 "Split the old READ_IMPLIES_EXEC workaround from executable
  PT_GNU_STACK now that toolchains long support PT_GNU_STACK marking and
  there's no need anymore to force modern programs into having all its
  user mappings executable instead of only the stack and the PROT_EXEC
  ones.

  Disable that automatic READ_IMPLIES_EXEC forcing on x86-64 and
  arm64.

  Add tables documenting how READ_IMPLIES_EXEC is handled on x86-64, arm
  and arm64.

  By Kees Cook"

* tag 'core_core_updates_for_5.8' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  arm64/elf: Disable automatic READ_IMPLIES_EXEC for 64-bit address spaces
  arm32/64/elf: Split READ_IMPLIES_EXEC from executable PT_GNU_STACK
  arm32/64/elf: Add tables to document READ_IMPLIES_EXEC
  x86/elf: Disable automatic READ_IMPLIES_EXEC on 64-bit
  x86/elf: Split READ_IMPLIES_EXEC from executable PT_GNU_STACK
  x86/elf: Add table to document READ_IMPLIES_EXEC

1 files changed, 23 insertions, 4 deletions

diff --git a/arch/arm/kernel/elf.c b/arch/arm/kernel/elf.c
index 182422981386..254ab7138c85 100644
--- a/arch/arm/kernel/elf.c
+++ b/arch/arm/kernel/elf.c
@@ -78,13 +78,32 @@ 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-stack |
+ * 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.
+ *  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
+ *   "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)
 {
-	if (executable_stack != EXSTACK_DISABLE_X)
+	if (executable_stack == EXSTACK_DEFAULT)
 		return 1;
 	if (cpu_architecture() < CPU_ARCH_ARMv6)
 		return 1;