Subject: Re: [PATCH v3 06/11] kselftest: arm64: fake_sigreturn_bad_magic

From: Dave Martin <[email-masked]>
Date: Tue, 13 Aug 2019 17:25:23 +0100

On Fri, Aug 02, 2019 at 06:02:55PM +0100, Cristian Marussi wrote:
> Added a simple fake_sigreturn testcase which builds a ucontext_t

Add

> with a bad magic header and place it onto the stack.
> Expects a SIGSEGV on test PASS.
> This commit also introduces a common utility assembly function to
> invoke a sigreturn using a fake provided sigframe.
> 
> Signed-off-by: Cristian Marussi <[email-masked]>
> ---
>  tools/testing/selftests/arm64/signal/Makefile |  2 +-
>  .../testing/selftests/arm64/signal/signals.S  | 64 +++++++++++++++++++
>  .../arm64/signal/test_signals_utils.h         |  1 +
>  .../arm64/signal/testcases/.gitignore         |  1 +
>  .../testcases/fake_sigreturn_bad_magic.c      | 63 ++++++++++++++++++
>  5 files changed, 130 insertions(+), 1 deletion(-)
>  create mode 100644 tools/testing/selftests/arm64/signal/signals.S
>  create mode 100644 tools/testing/selftests/arm64/signal/testcases/fake_sigreturn_bad_magic.c
> 
> diff --git a/tools/testing/selftests/arm64/signal/Makefile b/tools/testing/selftests/arm64/signal/Makefile
> index 8c8d08be4b0d..b3dcf315b5a4 100644
> --- a/tools/testing/selftests/arm64/signal/Makefile
> +++ b/tools/testing/selftests/arm64/signal/Makefile
> @@ -80,7 +80,7 @@ endif
>  # Common test-unit targets to build common-layout test-cases executables
>  # Needs secondary expansion to properly include the testcase c-file in pre-reqs
>  .SECONDEXPANSION:
> -$(PROGS): test_signals.c test_signals_utils.c testcases/testcases.c $$@.c test_signals.h test_signals_utils.h testcases/testcases.h
> +$(PROGS): test_signals.c test_signals_utils.c testcases/testcases.c signals.S $$@.c test_signals.h test_signals_utils.h testcases/testcases.h
>  	@if [ ! -d $(khdr_dir) ]; then \
>  		echo -n "\n!!! WARNING: $(khdr_dir) NOT FOUND."; \
>  		echo "===>  Are you sure Kernel Headers have been installed properly ?\n"; \
> diff --git a/tools/testing/selftests/arm64/signal/signals.S b/tools/testing/selftests/arm64/signal/signals.S
> new file mode 100644
> index 000000000000..6262b877400b
> --- /dev/null
> +++ b/tools/testing/selftests/arm64/signal/signals.S
> @@ -0,0 +1,64 @@
> +/*
> + * SPDX-License-Identifier: GPL-2.0
> + * Copyright (C) 2019 ARM Limited
> + */
> +
> +#include <asm/unistd.h>
> +
> +.section        ".rodata", "a"

The section name is not usually quoted in .section (though I guess it
works).

> +call_fmt:
> +	.asciz "Calling sigreturn with fake sigframe sized:%zd at calculated SP @%08lX\n"
> +
> +.text
> +
> +.globl fake_sigreturn
> +
> +/*	fake_sigreturn	x0:&sigframe,  x1:sigframe_size,  x2:alignment_SP */
> +fake_sigreturn:
> +	mov x20, x0
> +	mov x21, x1
> +	mov x22, x2
> +	mov x23, sp
> +
> +	/* create space on the stack for fake sigframe..."x22"-aligned */
> +	mov x0, #0

Why to we move 0 into x0?  We clobber x0 again in the next instruction:

> +	add x0, x21, x22
> +	sub x22, x22, #1
> +	bic x0, x0, x22
> +	sub x23, x23, x0

Can you explain the logic here?  I'm not sure I understand what this is
trying to do exactly.

I notice you further modify this in patch 11 -- see my comments there,
where I also suggest an alternative way of specifying a misaligned frame
that might be simpler.

> +
> +	ldr x0, =call_fmt
> +	mov x1, x21
> +	mov x2, x23
> +	bl printf
> +
> +	mov sp, x23
> +
> +	/* now fill it with the provided content... */
> +	mov x0, sp
> +	mov x1, x20
> +	mov x2, x21
> +	bl memcpy
> +
> +	/*
> +	 * Here saving a last minute SP to current->token acts as a marker:
> +	 * if we got here, we are successfully faking a sigreturn; in other
> +	 * words we are sure no bad fatal signal has been raised till now
> +	 * for unrelated reasons, so we should consider the possibl observed

typo: possible (or possibly?)

> +	 * fatal signal like SEGV coming from Kernel restore_sigframe() and
> +	 * triggered as expected from our test-case.
> +	 * For simplicity this assumes that current field 'token' is laid out
> +	 * as first in struct tdescr
> +	 */
> +	ldr x0, current
> +	str x23, [x0]
> +	/* SP is already pointing back to the just built fake sigframe here */
> +	mov x8, #__NR_rt_sigreturn
> +	svc #0
> +
> +	/*
> +	 * Above sigreturn should not return...looping here leads to a timeout
> +	 * and ensure proper and clean test failure, instead of jumping around
> +	 * on a potentially corrupted stack.
> +	 */
> +	b .
> diff --git a/tools/testing/selftests/arm64/signal/test_signals_utils.h b/tools/testing/selftests/arm64/signal/test_signals_utils.h
> index ce35be8ebc8e..2a71da7e6695 100644
> --- a/tools/testing/selftests/arm64/signal/test_signals_utils.h
> +++ b/tools/testing/selftests/arm64/signal/test_signals_utils.h
> @@ -12,4 +12,5 @@ int test_run(struct tdescr *td);
>  void test_result(struct tdescr *td);
>  
>  bool get_current_context(struct tdescr *td, ucontext_t *dest_uc);
> +int fake_sigreturn(void *sigframe, size_t sz, int alignment);
>  #endif
> diff --git a/tools/testing/selftests/arm64/signal/testcases/.gitignore b/tools/testing/selftests/arm64/signal/testcases/.gitignore
> index a48a118b1a1a..0ea6fdc3765c 100644
> --- a/tools/testing/selftests/arm64/signal/testcases/.gitignore
> +++ b/tools/testing/selftests/arm64/signal/testcases/.gitignore
> @@ -4,3 +4,4 @@ mangle_pstate_invalid_mode_el1
>  mangle_pstate_invalid_mode_el2
>  mangle_pstate_invalid_mode_el3
>  mangle_pstate_ssbs_regs
> +fake_sigreturn_bad_magic
> diff --git a/tools/testing/selftests/arm64/signal/testcases/fake_sigreturn_bad_magic.c b/tools/testing/selftests/arm64/signal/testcases/fake_sigreturn_bad_magic.c
> new file mode 100644
> index 000000000000..b4c063e02a7a
> --- /dev/null
> +++ b/tools/testing/selftests/arm64/signal/testcases/fake_sigreturn_bad_magic.c
> @@ -0,0 +1,63 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +/* Copyright (C) 2019 ARM Limited */
> +

We should probably also include <signal.h> here, since we're using
siginfo_t etc.

> +#include <stdio.h>
> +#include <ucontext.h>
> +
> +#include "test_signals_utils.h"
> +#include "testcases.h"
> +
> +struct fake_sigframe sf;
> +
> +static int fake_sigreturn_bad_magic_run(struct tdescr *td,
> +					siginfo_t *si, ucontext_t *uc)
> +{
> +	size_t resv_sz, offset;
> +	struct _aarch64_ctx *shead = GET_SF_RESV_HEAD(sf), *head;
> +
> +	/* just to fill the ucontext_t with something real */
> +	if (!get_current_context(td, &sf.uc))
> +		return 1;
> +
> +	resv_sz = GET_SF_RESV_SIZE(sf);
> +	/*
> +	 * find the terminator, preserving existing headers
> +	 * and verify amount of spare room in __reserved area.
> +	 */
> +	head = get_terminator(shead, resv_sz, &offset);
> +	/*
> +	 * try stripping extra_context header when low on space:
> +	 * we need at least 2*HDR_SZ space ... one for the KSFT_BAD_MAGIC
> +	 * and the other for the usual terminator.
> +	 */
> +	if (head && resv_sz - offset < HDR_SZ * 2) {

Can we factor out this logic for finding space in the signal frame?

We do pretty much the same thing in all the fake_sigreturn tests...

> +		fprintf(stderr, "Low on space:%zd. Discarding extra_context.\n",
> +			resv_sz - offset);
> +		head = get_header(shead, EXTRA_MAGIC, resv_sz, &offset);
> +	}
> +	/* just give up and timeout if still not enough space */

Do we actually time out?  I don't see where we actually wait, so doesn't
test_run() just fail immediately?

The same applies to all the other fake_sigreturn tests too.

> +	if (head && resv_sz - offset >= HDR_SZ) {

Should this be HDR_SZ * 2 again?  We need space for the face header and
space to write a terminator after it.

> +		fprintf(stderr, "Mangling template header. Spare space:%zd\n",
> +			resv_sz - offset);
> +		/*
> +		 * use a well known NON existent bad magic...something
> +		 * we should pretty sure won't be ever defined in Kernel
> +		 */
> +		head->magic = KSFT_BAD_MAGIC;
> +		head->size = HDR_SZ;
> +		write_terminator_record(GET_RESV_NEXT_HEAD(head));
> +
> +		ASSERT_BAD_CONTEXT(&sf.uc);
> +		fake_sigreturn(&sf, sizeof(sf), 16);
> +	}
> +
> +	return 1;
> +}

[...]

Cheers
---Dave

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