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Message-Id: <20200225051307.6401-2-keescook@chromium.org>
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
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