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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
This commit is contained in:
Kees Cook 2020-03-26 23:48:15 -07:00 committed by Borislav Petkov
parent ae83d0b416
commit 9d9e435f3f
1 changed files with 19 additions and 0 deletions
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19
arch/x86/include/asm/elf.h
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@ -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)