Subject: Re: [PATCH v7 06/11] kselftest: arm64: fake_sigreturn_bad_magic

From: Dave Martin <[email-masked]>
Date: Tue, 8 Oct 2019 19:01:02 +0100

On Mon, Oct 07, 2019 at 07:29:49pm +0100, Cristian Marussi wrote:
> Add a simple fake_sigreturn testcase which builds a ucontext_t with a bad
> magic header and place it onto the stack. Expects a SIGSEGV on test PASS.
> 
> Introduce a common utility assembly trampoline function to invoke a
> sigreturn while placing the provided sigframe at wanted alignment and
> also an helper to make space when needed inside the sigframe reserved
> area.
> 
> Signed-off-by: Cristian Marussi <[email-masked]>

Looks good to me now:

Reviewed-by: Dave Martin <[email-masked]>

Cheers
---Dave

> ---
> v6 --> v7
> - fix fake_sigreturn alignment routines (+15)
> v5 --> v6
> - fake_sigreturn: cleaned up, avoiding excessive SP misalignments
> - fake_sigreturn: better formatting and prologue
> - get_starting_header: cleand up and commented
> - avoid timeout on failure
> v3 --> v4
> - fix commit
> - fix signal.S, handle misalign requests too
> - remove unneeded comments
> - add signal.h include
> - added get_starting_head() helper
> - added test description
> ---
>  tools/testing/selftests/arm64/signal/Makefile |  2 +-
>  .../testing/selftests/arm64/signal/signals.S  | 64 +++++++++++++++++++
>  .../arm64/signal/test_signals_utils.h         |  2 +
>  .../testcases/fake_sigreturn_bad_magic.c      | 52 +++++++++++++++
>  .../arm64/signal/testcases/testcases.c        | 46 +++++++++++++
>  .../arm64/signal/testcases/testcases.h        |  4 ++
>  6 files changed, 169 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 f78f5190e3d4..b497cfea4643 100644
> --- a/tools/testing/selftests/arm64/signal/Makefile
> +++ b/tools/testing/selftests/arm64/signal/Makefile
> @@ -28,5 +28,5 @@ clean:
>  # 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
>  	$(CC) $(CFLAGS) $^ -o $@
> diff --git a/tools/testing/selftests/arm64/signal/signals.S b/tools/testing/selftests/arm64/signal/signals.S
> new file mode 100644
> index 000000000000..9f8c1aefc3b9
> --- /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"
> +call_fmt:
> +	.asciz "Calling sigreturn with fake sigframe sized:%zd at SP @%08lX\n"
> +
> +.text
> +
> +.globl fake_sigreturn
> +
> +/*	fake_sigreturn	x0:&sigframe,  x1:sigframe_size,  x2:misalign_bytes */
> +fake_sigreturn:
> +	stp	x29, x30, [sp, #-16]!
> +	mov	x29, sp
> +
> +	mov	x20, x0
> +	mov	x21, x1
> +	mov	x22, x2
> +
> +	/* create space on the stack for fake sigframe 16 bytes-aligned */
> +	add	x0, x21, x22
> +	add	x0, x0, #15
> +	bic	x0, x0, #15 /* round_up(sigframe_size + misalign_bytes, 16) */
> +	sub	sp, sp, x0
> +	add	x23, sp, x22 /* new sigframe base with misaligment if any */
> +
> +	ldr	x0, =call_fmt
> +	mov	x1, x21
> +	mov	x2, x23
> +	bl	printf
> +
> +	/* memcpy the provided content, while still keeping SP aligned */
> +	mov	x0, x23
> +	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 possibly observed
> +	 * 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]
> +	/* finally move SP to misaligned address...if any requested */
> +	mov	sp, x23
> +
> +	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 fd67b1f23c41..6772b5c8d274 100644
> --- a/tools/testing/selftests/arm64/signal/test_signals_utils.h
> +++ b/tools/testing/selftests/arm64/signal/test_signals_utils.h
> @@ -115,4 +115,6 @@ static __always_inline bool get_current_context(struct tdescr *td,
>  
>  	return td->live_uc_valid;
>  }
> +
> +int fake_sigreturn(void *sigframe, size_t sz, int misalign_bytes);
>  #endif
> 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..8dc600a7d4fd
> --- /dev/null
> +++ b/tools/testing/selftests/arm64/signal/testcases/fake_sigreturn_bad_magic.c
> @@ -0,0 +1,52 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * Copyright (C) 2019 ARM Limited
> + *
> + * Place a fake sigframe on the stack including a BAD Unknown magic
> + * record: on sigreturn Kernel must spot this attempt and the test
> + * case is expected to be terminated via SEGV.
> + */
> +
> +#include <signal.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)
> +{
> +	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;
> +
> +	/* need at least 2*HDR_SZ space: KSFT_BAD_MAGIC + terminator. */
> +	head = get_starting_head(shead, HDR_SZ * 2, GET_SF_RESV_SIZE(sf), NULL);
> +	if (!head)
> +		return 0;
> +
> +	/*
> +	 * 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), 0);
> +
> +	return 1;
> +}
> +
> +struct tdescr tde = {
> +		.name = "FAKE_SIGRETURN_BAD_MAGIC",
> +		.descr = "Trigger a sigreturn with a sigframe with a bad magic",
> +		.sig_ok = SIGSEGV,
> +		.timeout = 3,
> +		.run = fake_sigreturn_bad_magic_run,
> +};
> diff --git a/tools/testing/selftests/arm64/signal/testcases/testcases.c b/tools/testing/selftests/arm64/signal/testcases/testcases.c
> index 1914a01222a1..e3521949b800 100644
> --- a/tools/testing/selftests/arm64/signal/testcases/testcases.c
> +++ b/tools/testing/selftests/arm64/signal/testcases/testcases.c
> @@ -148,3 +148,49 @@ bool validate_reserved(ucontext_t *uc, size_t resv_sz, char **err)
>  
>  	return true;
>  }
> +
> +/*
> + * This function walks through the records inside the provided reserved area
> + * trying to find enough space to fit @need_sz bytes: if not enough space is
> + * available and an extra_context record is present, it throws away the
> + * extra_context record.
> + *
> + * It returns a pointer to a new header where it is possible to start storing
> + * our need_sz bytes.
> + *
> + * @shead: points to the start of reserved area
> + * @need_sz: needed bytes
> + * @resv_sz: reserved area size in bytes
> + * @offset: if not null, this will be filled with the offset of the return
> + *	    head pointer from @shead
> + *
> + * @return: pointer to a new head where to start storing need_sz bytes, or
> + *	    NULL if space could not be made available.
> + */
> +struct _aarch64_ctx *get_starting_head(struct _aarch64_ctx *shead,
> +				       size_t need_sz, size_t resv_sz,
> +				       size_t *offset)
> +{
> +	size_t offs = 0;
> +	struct _aarch64_ctx *head;
> +
> +	head = get_terminator(shead, resv_sz, &offs);
> +	/* not found a terminator...no need to update offset if any */
> +	if (!head)
> +		return head;
> +	if (resv_sz - offs < need_sz) {
> +		fprintf(stderr, "Low on space:%zd. Discarding extra_context.\n",
> +			resv_sz - offs);
> +		head = get_header(shead, EXTRA_MAGIC, resv_sz, &offs);
> +		if (!head || resv_sz - offs < need_sz) {
> +			fprintf(stderr,
> +				"Failed to reclaim space on sigframe.\n");
> +			return NULL;
> +		}
> +	}
> +
> +	fprintf(stderr, "Available space:%zd\n", resv_sz - offs);
> +	if (offset)
> +		*offset = offs;
> +	return head;
> +}
> diff --git a/tools/testing/selftests/arm64/signal/testcases/testcases.h b/tools/testing/selftests/arm64/signal/testcases/testcases.h
> index 04987f7870bc..ad884c135314 100644
> --- a/tools/testing/selftests/arm64/signal/testcases/testcases.h
> +++ b/tools/testing/selftests/arm64/signal/testcases/testcases.h
> @@ -97,4 +97,8 @@ static inline void write_terminator_record(struct _aarch64_ctx *tail)
>  		tail->size = 0;
>  	}
>  }
> +
> +struct _aarch64_ctx *get_starting_head(struct _aarch64_ctx *shead,
> +				       size_t need_sz, size_t resv_sz,
> +				       size_t *offset);
>  #endif
> -- 
> 2.17.1
> 
> 
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