[PATCH] fs: Preventing READ_IMPLIES_EXEC Propagation

On 2017/4/19 18:33, Catalin Marinas wrote:

On Tue, Apr 18, 2017 at 09:01:52PM +0100, Peter Maydell wrote:

quoted

On 18 April 2017 at 18:01, Catalin Marinas [off-list ref] wrote:

quoted

On Thu, Apr 13, 2017 at 08:33:52PM +0800, dongbo (E) wrote:

quoted

From: Dong Bo <redacted>

In load_elf_binary(), once the READ_IMPLIES_EXEC flag is set,
the flag is propagated to its child processes, even the elf
files are marked as not requiring executable stack. It may
cause superfluous operations on some arch, e.g.
__sync_icache_dcache on aarch64 due to a PROT_READ mmap is
also marked as PROT_EXEC.

Signed-off-by: Dong Bo <redacted>
---
 fs/binfmt_elf.c       | 2 ++
 fs/binfmt_elf_fdpic.c | 2 ++
 2 files changed, 4 insertions(+)

diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
index 5075fd5..c52e670 100644
--- a/fs/binfmt_elf.c
+++ b/fs/binfmt_elf.c

@@ -863,6 +863,8 @@ static int load_elf_binary(struct linux_binprm *bprm)
 	SET_PERSONALITY2(loc->elf_ex, &arch_state);
 	if (elf_read_implies_exec(loc->elf_ex, executable_stack))
 		current->personality |= READ_IMPLIES_EXEC;
+	else
+		current->personality &= ~READ_IMPLIES_EXEC;
  	if (!(current->personality & ADDR_NO_RANDOMIZE) && randomize_va_space)

		current->flags |= PF_RANDOMIZE;

diff --git a/fs/binfmt_elf_fdpic.c b/fs/binfmt_elf_fdpic.c
index cf93a4f..c4bc4d0 100644
--- a/fs/binfmt_elf_fdpic.c
+++ b/fs/binfmt_elf_fdpic.c

@@ -354,6 +354,8 @@ static int load_elf_fdpic_binary(struct linux_binprm *bprm)
 		set_personality(PER_LINUX);
 	if (elf_read_implies_exec(&exec_params.hdr, executable_stack))
 		current->personality |= READ_IMPLIES_EXEC;
+	else
+		current->personality &= ~READ_IMPLIES_EXEC;
  	setup_new_exec(bprm);

quoted

That's affecting most architectures with a risk of ABI breakage. We
could do it on arm64 only, though I'm not yet clear on the ABI
implications (at a first look, there shouldn't be any).

Is there a reason why it isn't just straightforwardly a bug
(which we could fix) to make READ_IMPLIES_EXEC propagate to
child processes?

While I agree that it looks like a bug, if there are user programs
relying on such bug we call it "ABI". On arm64, I don't think there is
anything relying on inheriting READ_IMPLIES_EXEC but I wouldn't change
the compat task handling without the corresponding change in arch/arm.

With READ_IMPLIES_EXEC propagation, several hundreds times of
__sync_icache_dcache operations shows up than not READ_IMPLIES_EXEC
propagation, which degenerating the system performance. Changing arm64
only would settle our problem down, thanks for figuring out previously.

Seems that arch/arm had discussed the propagation of READ_IMPLIES_EXEC:
http://lists.infradead.org/pipermail/linux-arm-kernel/2012-February/086490.html
But the READ_IMPLIES_EXEC is still not cleared in elf_set_personality().

quoted

AFAICT this should be per-process: just because
init happens not to have been (re)compiled to permit non-executable
stacks doesn't mean every process on the system needs to have
an executable stack.

I think this also affects the heap if brk(2) is used (via
VM_DATA_DEFAULT_FLAGS though I guess malloc mostly uses mmap these
days).

quoted

Behaviour shouldn't be variable across architectures either, I would
hope.

The behaviour has already been variable for a long time. Even on x86,
AFAICT x86_32 differs from x86_64 in this respect.

Anyway, the patch should be posted to linux-arch for a cross-arch
discussion.

OK, this mail Cc to linux-arch. Thanks.