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Subject[tip: core/core] x86/elf: Add table to document READ_IMPLIES_EXEC
The following commit has been merged into the core/core branch of tip:

Commit-ID: 9d9e435f3f2492bfd196acacb61cc9a9212d8170
Gitweb: https://git.kernel.org/tip/9d9e435f3f2492bfd196acacb61cc9a9212d8170
Author: Kees Cook <keescook@chromium.org>
AuthorDate: Thu, 26 Mar 2020 23:48:15 -07:00
Committer: Borislav Petkov <bp@suse.de>
CommitterDate: Mon, 20 Apr 2020 15:28:40 +02:00

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
---
arch/x86/include/asm/elf.h | 19 +++++++++++++++++++
1 file changed, 19 insertions(+)

diff --git a/arch/x86/include/asm/elf.h b/arch/x86/include/asm/elf.h
index 69c0f89..ee459d4 100644
--- 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)
\
 
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