linux-kernel - [PATCH v4 1/6] x86/elf: Add table to document READ_IMPLIES

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Date:   Mon, 24 Feb 2020 21:13:02 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Kees Cook <keescook@...omium.org>,
        Jason Gunthorpe <jgg@...lanox.com>,
        Hector Marco-Gisbert <hecmargi@....es>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Catalin Marinas <catalin.marinas@....com>,
        Russell King <linux@...linux.org.uk>,
        Will Deacon <will@...nel.org>, Jann Horn <jannh@...gle.com>,
        x86@...nel.org, linux-arm-kernel@...ts.infradead.org,
        kernel-hardening@...ts.openwall.com, linux-kernel@...r.kernel.org
Subject: [PATCH v4 1/6] 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@...omium.org>
Reviewed-by: Jason Gunthorpe <jgg@...lanox.com>
---
 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 69c0f892e310..733f69c2b053 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 GNU_STACK | exec-all   | exec-all         | exec-all       |
+ * GNU_STACK == RWX  | exec-all   | exec-all         | exec-all       |
+ * 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)
-- 
2.20.1