[PATCH v8 11/20] aarch64: Enable GCS in dynamic linked exe
Adhemerval Zanella Netto
adhemerval.zanella@linaro.org
Wed Jan 15 14:15:33 GMT 2025
On 14/01/25 13:03, Yury Khrustalev wrote:
> From: Szabolcs Nagy <szabolcs.nagy@arm.com>
>
> Use the dynamic linker start code to enable GCS in the dynamic linked
> case after _dl_start returns and before _dl_start_user which marks
> the point after which user code may run.
>
> Like in the static linked case this ensures that GCS is enabled on a
> top level stack frame.
LGTM, thanks.
Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>
> ---
> sysdeps/aarch64/Makefile | 8 ++++--
> sysdeps/aarch64/dl-gcs.c | 25 ++++++++++++++++++
> sysdeps/aarch64/dl-start.S | 34 ++++++++++++++++++++++---
> sysdeps/aarch64/rtld-global-offsets.sym | 5 ++++
> 4 files changed, 66 insertions(+), 6 deletions(-)
> create mode 100644 sysdeps/aarch64/dl-gcs.c
>
> diff --git a/sysdeps/aarch64/Makefile b/sysdeps/aarch64/Makefile
> index 141d7d9cc2..74479604f2 100644
> --- a/sysdeps/aarch64/Makefile
> +++ b/sysdeps/aarch64/Makefile
> @@ -9,7 +9,9 @@ LDFLAGS-rtld += -Wl,-z,force-bti,--fatal-warnings
> endif
>
> ifeq ($(subdir),elf)
> -sysdep-dl-routines += dl-bti
> +sysdep-dl-routines += \
> + dl-bti \
> + dl-gcs
>
> tests += tst-audit26 \
> tst-audit27
> @@ -35,7 +37,9 @@ endif
> ifeq ($(subdir),elf)
> sysdep-rtld-routines += dl-start
> sysdep-dl-routines += tlsdesc dl-tlsdesc
> -gen-as-const-headers += dl-link.sym
> +gen-as-const-headers += \
> + dl-link.sym \
> + rtld-global-offsets.sym
>
> tests-internal += tst-ifunc-arg-1 tst-ifunc-arg-2
>
> diff --git a/sysdeps/aarch64/dl-gcs.c b/sysdeps/aarch64/dl-gcs.c
> new file mode 100644
> index 0000000000..870336f2bb
> --- /dev/null
> +++ b/sysdeps/aarch64/dl-gcs.c
> @@ -0,0 +1,25 @@
> +/* AArch64 GCS functions.
> + Copyright (C) 2024-2025 Free Software Foundation, Inc.
> +
> + The GNU C Library is free software; you can redistribute it and/or
> + modify it under the terms of the GNU Lesser General Public
> + License as published by the Free Software Foundation; either
> + version 2.1 of the License, or (at your option) any later version.
> +
> + The GNU C Library is distributed in the hope that it will be useful,
> + but WITHOUT ANY WARRANTY; without even the implied warranty of
> + MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
> + Lesser General Public License for more details.
> +
> + You should have received a copy of the GNU Lesser General Public
> + License along with the GNU C Library; if not, see
> + <https://www.gnu.org/licenses/>. */
> +
> +#include <ldsodefs.h>
> +
> +/* Used to report error when prctl system call to enabled GCS fails. */
> +
> +void _dl_gcs_enable_failed (int code)
> +{
> + _dl_fatal_printf ("failed to enable GCS: %d\n", -code);
> +}
> diff --git a/sysdeps/aarch64/dl-start.S b/sysdeps/aarch64/dl-start.S
> index 3b8eff0acd..a249fda21c 100644
> --- a/sysdeps/aarch64/dl-start.S
> +++ b/sysdeps/aarch64/dl-start.S
> @@ -18,6 +18,7 @@
> <https://www.gnu.org/licenses/>. */
>
> #include <sysdep.h>
> +#include <rtld-global-offsets.h>
>
> ENTRY (_start)
> /* Create an initial frame with 0 LR and FP */
> @@ -25,11 +26,33 @@ ENTRY (_start)
> mov x29, #0
> mov x30, #0
>
> + /* Load and relocate all library dependencies. */
> mov x0, sp
> PTR_ARG (0)
> bl _dl_start
> /* Returns user entry point in x0. */
> mov PTR_REG (21), PTR_REG (0)
> +
> + /* Use GL(dl_aarch64_gcs) to set the shadow stack status. */
> + adrp x16, _rtld_local
> + add PTR_REG (16), PTR_REG (16), :lo12:_rtld_local
> + ldr x1, [x16, GL_DL_AARCH64_GCS_OFFSET]
> + cbz x1, L(skip_gcs_enable)
> +
> + /* Enable GCS before user code runs. Note that IFUNC resolvers and
> + LD_AUDIT hooks may run before, but should not create threads. */
We don't really prevent audit hooks to create threads, so we might revise this
placement later.
> +#define PR_SET_SHADOW_STACK_STATUS 75
> +#define PR_SHADOW_STACK_ENABLE (1UL << 0)
> + mov x0, PR_SET_SHADOW_STACK_STATUS
> + mov x1, PR_SHADOW_STACK_ENABLE
> + mov x2, 0
> + mov x3, 0
> + mov x4, 0
> + mov x8, #SYS_ify(prctl)
> + svc 0x0
> + cbnz w0, L(failed_gcs_enable)
> +L(skip_gcs_enable):
> +
> .globl _dl_start_user
> .type _dl_start_user, %function
> _dl_start_user:
> @@ -40,14 +63,17 @@ _dl_start_user:
> /* Compute envp. */
> add PTR_REG (3), PTR_REG (2), PTR_REG (1), lsl PTR_LOG_SIZE
> add PTR_REG (3), PTR_REG (3), PTR_SIZE
> - adrp x16, _rtld_local
> - add PTR_REG (16), PTR_REG (16), :lo12:_rtld_local
> + /* Run the init functions of the loaded modules. */
> ldr PTR_REG (0), [x16]
> bl _dl_init
> /* Load the finalizer function. */
> adrp x0, _dl_fini
> add PTR_REG (0), PTR_REG (0), :lo12:_dl_fini
> /* Jump to the user's entry point. */
> - mov x16, x21
> - br x16
> + mov x16, x21
> + br x16
> +
> +L(failed_gcs_enable):
> + b _dl_gcs_enable_failed
> +
> END (_start)
> diff --git a/sysdeps/aarch64/rtld-global-offsets.sym b/sysdeps/aarch64/rtld-global-offsets.sym
> index 23cdaf7d9e..6c0690bb95 100644
> --- a/sysdeps/aarch64/rtld-global-offsets.sym
> +++ b/sysdeps/aarch64/rtld-global-offsets.sym
> @@ -3,8 +3,13 @@
> #include <ldsodefs.h>
>
> #define GLRO_offsetof(name) offsetof (struct rtld_global_ro, _##name)
> +#define GL_offsetof(name) offsetof (struct rtld_global, _##name)
>
> -- Offsets of _rtld_global_ro in libc.so
>
> GLRO_DL_HWCAP_OFFSET GLRO_offsetof (dl_hwcap)
> GLRO_DL_HWCAP2_OFFSET GLRO_offsetof (dl_hwcap2)
> +
> +-- Offsets of _rtld_global in libc.so
> +
> +GL_DL_AARCH64_GCS_OFFSET GL_offsetof (dl_aarch64_gcs)
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