[PATCH v4] ARM: imx: Fix suspend/resume crash with Clang CFI
Nick Desaulniers
ndesaulniers at google.com
Fri Aug 28 11:29:42 PDT 2026
On Thu, Aug 27, 2026 at 12:46 PM Yo'av Moshe <linux at yoavmoshe.com> wrote:
>
> The suspend code that runs from OCRAM is copied there with fncpy(),
> which does not copy the kCFI type hash preceding the function. With
> CONFIG_CFI=y the indirect call through imx6_suspend_in_ocram_fn
> therefore panics.
>
> Keep the call covered by CFI instead of exempting it:
Nice! This is a pretty good progression.
>
> - Declare imx6_suspend() with SYM_TYPED_FUNC_START() so a type hash
> is emitted for it. fncpy() requires the entry point to be 8-byte
> aligned, and the macro's alignment applies to the hash rather than
> the entry that follows it, so pad manually before the macro.
>
> - Add a cfi_type member at the end of struct imx6_cpu_pm_info, which
> directly precedes the OCRAM copy of the function. It fits in the
> struct's tail padding, so no sizes or offsets change. Fill it using
> cfi_get_func_hash(), putting the hash where the caller's CFI check
> expects it: four bytes before the function entry.
>
> Also mark ccm_base, suspend_ocram_base and imx6_suspend_in_ocram_fn
> as __ro_after_init: they are only written during __init, and the
> function pointer in particular should not be writable afterwards.
>
> Suggested-by: Nick Desaulniers <ndesaulniers at google.com>
> Cc: stable at vger.kernel.org
> Signed-off-by: Yo'av Moshe <linux at yoavmoshe.com>
> ---
> Tested on a Kobo Clara HD (i.MX6SLL) running postmarketOS
> (clang/LLVM, CONFIG_CFI=y): suspend and resume work.
>
> Changes in v4, all following Nick's suggestions:
> - Drop the v3 __nocfi wrapper; keep the indirect call CFI-checked.
> - Store the hash in a cfi_type member in the struct's tail padding
> instead of open-coded pointer arithmetic.
> - Use cfi_get_func_hash() instead of reading the hash manually.
> - Use SYM_TYPED_FUNC_START()/SYM_FUNC_END() instead of ENTRY()/
> ENDPROC() with a hand-rolled __CFI_TYPE.
>
> Note: linux/uaccess.h is included before linux/cfi.h because
> cfi_get_func_hash() uses get_kernel_nofault() and cfi.h does not
> include uaccess.h itself.
Ah, no, we (you) should fix that. include/linux/cfi.h should IWYU.
>
> arch/arm/mach-imx/pm-imx6.c | 25 ++++++++++++++++++++++---
> arch/arm/mach-imx/suspend-imx6.S | 14 ++++++++++++--
> 2 files changed, 34 insertions(+), 5 deletions(-)
>
> diff --git a/arch/arm/mach-imx/pm-imx6.c b/arch/arm/mach-imx/pm-imx6.c
> index a671ca4..2d3d3cd 100644
> --- a/arch/arm/mach-imx/pm-imx6.c
> +++ b/arch/arm/mach-imx/pm-imx6.c
> @@ -4,6 +4,8 @@
> * Copyright 2011 Linaro Ltd.
> */
>
> +#include <linux/uaccess.h>
> +#include <linux/cfi.h>
This is a bug in include/linux/cfi.h; don't work around it. Fix it.
> #include <linux/clk/imx.h>
> #include <linux/delay.h>
> #include <linux/init.h>
> @@ -61,9 +63,9 @@
> #define MX6Q_SUSPEND_OCRAM_SIZE 0x1000
> #define MX6_MAX_MMDC_IO_NUM 33
>
> -static void __iomem *ccm_base;
> -static void __iomem *suspend_ocram_base;
> -static void (*imx6_suspend_in_ocram_fn)(void __iomem *ocram_vbase);
> +static void __iomem *ccm_base __ro_after_init;
> +static void __iomem *suspend_ocram_base __ro_after_init;
> +static void (*imx6_suspend_in_ocram_fn)(void __iomem *ocram_vbase) __ro_after_init;
>
> /*
> * suspend ocram space layout:
> @@ -229,8 +231,18 @@ struct imx6_cpu_pm_info {
> struct imx6_pm_base l2_base;
> u32 mmdc_io_num; /* Number of MMDC IOs which need saved/restored. */
> u32 mmdc_io_val[MX6_MAX_MMDC_IO_NUM][2]; /* To save offset and value */
> + u32 cfi_type; /* kCFI type hash of imx6_suspend() */
> } __aligned(8);
>
> +/*
> + * The ocram copy of imx6_suspend() starts right after struct imx6_cpu_pm_info,
> + * and the CFI check on the indirect call reads the kCFI type hash from the
> + * four bytes preceding the function entry, so cfi_type must occupy the last
> + * four bytes of the struct, i.e. fit into its tail padding.
> + */
> +static_assert(offsetofend(struct imx6_cpu_pm_info, cfi_type) ==
> + sizeof(struct imx6_cpu_pm_info));
on this IWYU thread, shall we also include linux/build_bug.h for
static_assert or linux/stddef.h for offsetofend?
> +
> void imx6_set_int_mem_clk_lpm(bool enable)
> {
> u32 val = readl_relaxed(ccm_base + CGPR);
> @@ -568,6 +580,13 @@ static int __init imx6q_suspend_init(const struct imx6_pm_socdata *socdata)
> mmdc_offset_array[i]);
> }
>
> + /*
> + * Mask out the Thumb bit, as cfi_get_func_hash() expects the
> + * function's actual start address. Returns 0 if CONFIG_CFI=n.
> + */
> + pm_info->cfi_type =
> + cfi_get_func_hash((void *)((uintptr_t)&imx6_suspend & ~1UL));
> +
> imx6_suspend_in_ocram_fn = fncpy(
> suspend_ocram_base + sizeof(*pm_info),
> &imx6_suspend,
> diff --git a/arch/arm/mach-imx/suspend-imx6.S b/arch/arm/mach-imx/suspend-imx6.S
> index 63ccc2d..f60c4e5 100644
> --- a/arch/arm/mach-imx/suspend-imx6.S
> +++ b/arch/arm/mach-imx/suspend-imx6.S
> @@ -3,6 +3,7 @@
> * Copyright 2014 Freescale Semiconductor, Inc.
> */
>
> +#include <linux/cfi_types.h>
> #include <linux/linkage.h>
> #include <asm/assembler.h>
> #include <asm/asm-offsets.h>
> @@ -148,7 +149,16 @@
>
> .endm
>
> -ENTRY(imx6_suspend)
> +#ifdef CONFIG_CFI
> + /*
> + * Pad the location counter so that the type hash emitted by
> + * SYM_TYPED_FUNC_START() below ends on an 8-byte boundary:
> + * fncpy() requires the function entry to be 8-byte aligned.
> + */
> + .align 3
> + .4byte 0
I still don't like these assembler directives inline like this; this
feels like we should have perhaps a new macro in
include/linux/cfi_types.h. Thoughts, Sami?
One that garuntees the 8B alignment of the symbol for w/e that
function patching routine requires?
> +#endif
> +SYM_TYPED_FUNC_START(imx6_suspend)
> ldr r1, [r0, #PM_INFO_PBASE_OFFSET]
> ldr r2, [r0, #PM_INFO_RESUME_ADDR_OFFSET]
> ldr r3, [r0, #PM_INFO_DDR_TYPE_OFFSET]
> @@ -329,4 +339,4 @@ resume:
> resume_mmdc
>
> ret lr
> -ENDPROC(imx6_suspend)
> +SYM_FUNC_END(imx6_suspend)
> --
> 2.55.0
>
--
Thanks,
~Nick Desaulniers
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