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@chromium.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Jason Gunthorpe <jgg@mellanox.com>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Link: https://lkml.kernel.org/r/20200327064820.12602-5-keescook@chromium.org

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)
 {

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