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Subject[tip: core/core] arm32/64/elf: Add tables to document READ_IMPLIES_EXEC
The following commit has been merged into the core/core branch of tip:

Commit-ID: 78066055b08096ed3282c027de9d9e137f9a1580
Gitweb: https://git.kernel.org/tip/78066055b08096ed3282c027de9d9e137f9a1580
Author: Kees Cook <keescook@chromium.org>
AuthorDate: Thu, 26 Mar 2020 23:48:18 -07:00
Committer: Borislav Petkov <bp@suse.de>
CommitterDate: Mon, 20 Apr 2020 19:41:50 +02:00

arm32/64/elf: Add tables to document READ_IMPLIES_EXEC

Add tables to document the current behavior of READ_IMPLIES_EXEC in
preparation for changing the behavior for both arm64 and arm.

Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Jason Gunthorpe <jgg@mellanox.com>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Link: https://lkml.kernel.org/r/20200327064820.12602-5-keescook@chromium.org
---
arch/arm/kernel/elf.c | 24 +++++++++++++++++++++---
arch/arm64/include/asm/elf.h | 20 ++++++++++++++++++++
2 files changed, 41 insertions(+), 3 deletions(-)

diff --git a/arch/arm/kernel/elf.c b/arch/arm/kernel/elf.c
index 1824229..5ccd4ac 100644
--- a/arch/arm/kernel/elf.c
+++ b/arch/arm/kernel/elf.c
@@ -78,9 +78,27 @@ void elf_set_personality(const struct elf32_hdr *x)
EXPORT_SYMBOL(elf_set_personality);

/*
- * Set READ_IMPLIES_EXEC if:
- * - the binary requires an executable stack
- * - we're running on a CPU which doesn't support NX.
+ * 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 |
+ * ELF:              |            |           |
+ * ---------------------|------------|------------|
+ * missing PT_GNU_STACK | exec-all   | exec-all  |
+ * PT_GNU_STACK == RWX  | exec-all   | exec-all  |
+ * PT_GNU_STACK == RW   | exec-all  | 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
+ *
*/
int arm_elf_read_implies_exec(int executable_stack)
{
diff --git a/arch/arm64/include/asm/elf.h b/arch/arm64/include/asm/elf.h
index b618017..986ecf4 100644
--- a/arch/arm64/include/asm/elf.h
+++ b/arch/arm64/include/asm/elf.h
@@ -96,6 +96,26 @@
*/
#define elf_check_arch(x) ((x)->e_machine == EM_AARCH64)

+/*
+ * 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*: | arm32    | arm64 |
+ * ELF:              |            |            |
+ * ---------------------|------------|------------|
+ * missing PT_GNU_STACK | exec-all   | exec-all   |
+ * PT_GNU_STACK == RWX  | exec-all   | exec-all   |
+ * PT_GNU_STACK == RW   | 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.
+ *
+ * *all arm64 CPUs support NX, so there is no "lacks NX" column.
+ *
+ */
#define elf_read_implies_exec(ex,stk) (stk != EXSTACK_DISABLE_X)

#define CORE_DUMP_USE_REGSET
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