From: Kees Cook <keescook@chromium.org>
To: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Cc: Kees Cook <keescook@chromium.org>,
Tiezhu Yang <yangtiezhu@loongson.cn>,
Xuefeng Li <lixuefeng@loongson.cn>,
Juxin Gao <gaojuxin@loongson.cn>,
linux-mips@vger.kernel.org, Nathan Chancellor <nathan@kernel.org>,
Nick Desaulniers <ndesaulniers@google.com>,
linux-kernel@vger.kernel.org, clang-built-linux@googlegroups.com,
linux-hardening@vger.kernel.org
Subject: [PATCH] MIPS: Modernize READ_IMPLIES_EXEC
Date: Wed, 1 Sep 2021 12:42:08 -0700 [thread overview]
Message-ID: <20210901194208.2420671-1-keescook@chromium.org> (raw)
I'm doing some thread necromancy of
https://lore.kernel.org/lkml/202007081624.82FA0CC1EA@keescook/
x86, arm64, and arm32 adjusted their READ_IMPLIES_EXEC logic to better
align with the safer defaults and the interactions with other mappings,
which I illustrated with this comment on x86:
/*
* 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-none |
* PT_GNU_STACK == RWX | exec-stack | exec-stack | exec-stack |
* 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.
* exec-stack: only the stack and 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
*
*/
For MIPS, the "lacks NX" above is the "!cpu_has_rixi" check. On x86,
we decided that the READ_IMPLIES_EXEC flag needed to reflect the
expectations, not the architectural behavior due to bad interactions
as noted above, as always returning "1" on non-NX hardware breaks
some mappings.
The other part of the issue is "what does the MIPS toolchain do for
PT_GNU_STACK?" The answer seems to be "by default, include PT_GNU_STACK,
but mark it executable" (likely due to concerns over non-NX hardware):
$ mipsel-linux-gnu-gcc -o hello_world hello_world.c
$ llvm-readelf -lW hellow_world | grep GNU_STACK
GNU_STACK 0x000000 0x00000000 0x00000000 0x00000 0x00000 RWE 0x10
Given that older hardware doesn't support non-executable memory, it
seems safe to make the "PT_GNU_STACK is absent" logic mean "assume
non-executable", but this might break very old software running on
modern MIPS. This situation matches the ia32-on-x86_64 logic x86
uses (which assumes needing READ_IMPLIES_EXEC in that situation). But
modern toolchains on modern MIPS hardware should follow a safer default
(assume NX stack).
A follow-up to this change would be to switch the MIPS toolchain to emit
a non-executable PT_GNU_STACK, as this seems to be unneeded.
Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Cc: Tiezhu Yang <yangtiezhu@loongson.cn>
Cc: Xuefeng Li <lixuefeng@loongson.cn>
Cc: Juxin Gao <gaojuxin@loongson.cn>
Cc: linux-mips@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
---
arch/mips/kernel/elf.c | 16 +++++-----------
1 file changed, 5 insertions(+), 11 deletions(-)
diff --git a/arch/mips/kernel/elf.c b/arch/mips/kernel/elf.c
index 7b045d2a0b51..5582a4ca1e9e 100644
--- a/arch/mips/kernel/elf.c
+++ b/arch/mips/kernel/elf.c
@@ -328,16 +328,10 @@ void mips_set_personality_nan(struct arch_elf_state *state)
int mips_elf_read_implies_exec(void *elf_ex, int exstack)
{
- if (exstack != EXSTACK_DISABLE_X) {
- /* The binary doesn't request a non-executable stack */
- return 1;
- }
-
- if (!cpu_has_rixi) {
- /* The CPU doesn't support non-executable memory */
- return 1;
- }
-
- return 0;
+ /*
+ * Set READ_IMPLIES_EXEC only on non-NX systems that
+ * do not request a specific state via PT_GNU_STACK.
+ */
+ return (!cpu_has_rixi && exstack == EXSTACK_DEFAULT);
}
EXPORT_SYMBOL(mips_elf_read_implies_exec);
--
2.30.2
next reply other threads:[~2021-09-01 19:42 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-09-01 19:42 Kees Cook [this message]
2022-02-22 23:49 ` [PATCH] MIPS: Modernize READ_IMPLIES_EXEC Kees Cook
2022-02-23 15:03 ` Thomas Bogendoerfer
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