Subject: Re: [PATCH v5 10/11] kselftest: arm64: fake_sigreturn_bad_size

From: Dave Martin <[email-masked]>
Date: Wed, 4 Sep 2019 12:49:33 +0100

On Mon, Sep 02, 2019 at 12:29:31pm +0100, Cristian Marussi wrote:
> Add a simple fake_sigreturn testcase which builds a ucontext_t with a
> badly sized header that causes a overrun in the __reserved area and
> place it onto the stack. Expects a SIGSEGV on test PASS.
> 
> Signed-off-by: Cristian Marussi <[email-masked]>
> ---
> v3 --> v4
> - fix commit
> - missing include
> - using new get_starting_head() helper
> - added test description
> ---
>  .../testcases/fake_sigreturn_bad_size.c       | 77 +++++++++++++++++++
>  1 file changed, 77 insertions(+)
>  create mode 100644 tools/testing/selftests/arm64/signal/testcases/fake_sigreturn_bad_size.c
> 
> diff --git a/tools/testing/selftests/arm64/signal/testcases/fake_sigreturn_bad_size.c b/tools/testing/selftests/arm64/signal/testcases/fake_sigreturn_bad_size.c
> new file mode 100644
> index 000000000000..b1156afdb691
> --- /dev/null
> +++ b/tools/testing/selftests/arm64/signal/testcases/fake_sigreturn_bad_size.c
> @@ -0,0 +1,77 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * Copyright (C) 2019 ARM Limited
> + *
> + * Place a fake sigframe on the stack including a bad record overflowing
> + * the __reserved space: 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;
> +
> +#define MIN_SZ_ALIGN	16
> +
> +static int fake_sigreturn_bad_size_run(struct tdescr *td,
> +				       siginfo_t *si, ucontext_t *uc)
> +{
> +	size_t resv_sz, need_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);
> +	/* at least HDR_SZ + bad sized esr_context needed */
> +	need_sz = HDR_SZ + sizeof(struct esr_context);

Nit: can we write this sum the other way round (see comment on patch 9)?

> +	head = get_starting_head(shead, need_sz, resv_sz, &offset);
> +	if (head) {
> +		/*
> +		 * Use an esr_context to build a fake header with a
> +		 * size greater then the free __reserved area minus HDR_SZ;
> +		 * using ESR_MAGIC here since it is not checked for size nor
> +		 * is limited to one instance.
> +		 *
> +		 * At first inject an additional normal esr_context
> +		 */
> +		head->magic = ESR_MAGIC;
> +		head->size = sizeof(struct esr_context);
> +		/* and terminate properly */
> +		write_terminator_record(GET_RESV_NEXT_HEAD(head));
> +		ASSERT_GOOD_CONTEXT(&sf.uc);
> +
> +		/*
> +		 * now mess with fake esr_context size: leaving less space than
> +		 * needed while keeping size value 16-aligned
> +		 *
> +		 * It must trigger a SEGV from Kernel on:
> +		 *
> +		 *	resv_sz - offset < sizeof(*head)
> +		 */
> +		/* at first set the maximum good 16-aligned size */
> +		head->size =
> +			(resv_sz - offset - need_sz + MIN_SZ_ALIGN) & ~0xfUL;
> +		/* plus a bit more of 16-aligned sized stuff */
> +		head->size += MIN_SZ_ALIGN;

Can we also have versions of this test that try:

 a) a size that doesn't overflow __reserved[], but is not a multiple of 16
 b) a size that is less than 16
 c) a size that does overflow __reserved[], but by less than 16 bytes?

These tests are all closely related and can probably be macro-ised
easily.  They can go on the TODO list for now anyway: let's get this
series settled in its current form first.

In any case:

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

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