x86/elf: Add table to document READ_IMPLIES_EXEC

Add a table to document the current behavior of READ_IMPLIES_EXEC in
preparation for changing the behavior.

Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Jason Gunthorpe <jgg@mellanox.com>
Link: https://lkml.kernel.org/r/20200327064820.12602-2-keescook@chromium.org

diff --git a/arch/x86/include/asm/elf.h b/arch/x86/include/asm/elf.h
index 69c0f892e310a37a9f8bd543227a73104616b5df..ee459d4c3b45224b30da3553c762ecf1424e67c6 100644 (file)
--- a/arch/x86/include/asm/elf.h
+++ b/arch/x86/include/asm/elf.h
@@ -281,6 +281,25 @@ extern u32 elf_hwcap2;
 /*
  * 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, ia32     | has NX, x86_64 |
+ * ELF:                 |            |                  |                |
+ * ---------------------|------------|------------------|----------------|
+ * missing PT_GNU_STACK | exec-all   | exec-all         | exec-all       |
+ * PT_GNU_STACK == RWX  | exec-all   | exec-all         | exec-all       |
+ * 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.
+ *
+ *  *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
+ *
  */
 #define elf_read_implies_exec(ex, executable_stack)    \
        (executable_stack != EXSTACK_DISABLE_X)