Re: [PATCH mm v4 32/39] kasan, arm64: don't tag executable vmalloc allocations

Andrey Konovalov
2021-12-20T22:05:17Z

On Mon, Dec 20, 2021 at 11:02 PM <andrey.konovalov@linux.dev> wrote:
>
> From: Andrey Konovalov <andreyknvl@google.com>
>
> Besides asking vmalloc memory to be executable via the prot argument
> of __vmalloc_node_range() (see the previous patch), the kernel can skip
> that bit and instead mark memory as executable via set_memory_x().
>
> Once tag-based KASAN modes start tagging vmalloc allocations, executing
> code from such allocations will lead to the PC register getting a tag,
> which is not tolerated by the kernel.
>
> Generic kernel code typically allocates memory via module_alloc() if
> it intends to mark memory as executable. (On arm64 module_alloc()
> uses __vmalloc_node_range() without setting the executable bit).
>
> Thus, reset pointer tags of pointers returned from module_alloc().
>
> However, on arm64 there's an exception: the eBPF subsystem. Instead of
> using module_alloc(), it uses vmalloc() (via bpf_jit_alloc_exec())
> to allocate its JIT region.
>
> Thus, reset pointer tags of pointers returned from bpf_jit_alloc_exec().
>
> Resetting tags for these pointers results in untagged pointers being
> passed to set_memory_x(). This causes conflicts in arithmetic checks
> in change_memory_common(), as vm_struct->addr pointer returned by
> find_vm_area() is tagged.
>
> Reset pointer tag of find_vm_area(addr)->addr in change_memory_common().
>
> Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
>
> ---
>
> Changes v3->v4:
> - Reset pointer tag in change_memory_common().
>
> Changes v2->v3:
> - Add this patch.
> ---
> arch/arm64/kernel/module.c | 3 ++-
> arch/arm64/mm/pageattr.c | 2 +-
> arch/arm64/net/bpf_jit_comp.c | 3 ++-
> 3 files changed, 5 insertions(+), 3 deletions(-)
>
> diff --git a/arch/arm64/kernel/module.c b/arch/arm64/kernel/module.c
> index d3a1fa818348..f2d4bb14bfab 100644
> --- a/arch/arm64/kernel/module.c
> +++ b/arch/arm64/kernel/module.c
> @@ -63,7 +63,8 @@ void *module_alloc(unsigned long size)
> return NULL;
> }
>
> - return p;
> + /* Memory is intended to be executable, reset the pointer tag. */
> + return kasan_reset_tag(p);
> }
>
> enum aarch64_reloc_op {
> diff --git a/arch/arm64/mm/pageattr.c b/arch/arm64/mm/pageattr.c
> index a3bacd79507a..64e985eaa52d 100644
> --- a/arch/arm64/mm/pageattr.c
> +++ b/arch/arm64/mm/pageattr.c
> @@ -85,7 +85,7 @@ static int change_memory_common(unsigned long addr, int numpages,
> */
> area = find_vm_area((void *)addr);
> if (!area ||
> - end > (unsigned long)area->addr + area->size ||
> + end > (unsigned long)kasan_reset_tag(area->addr) + area->size ||
> !(area->flags & VM_ALLOC))
> return -EINVAL;
>
> diff --git a/arch/arm64/net/bpf_jit_comp.c b/arch/arm64/net/bpf_jit_comp.c
> index 07aad85848fa..381a67922c2d 100644
> --- a/arch/arm64/net/bpf_jit_comp.c
> +++ b/arch/arm64/net/bpf_jit_comp.c
> @@ -1147,7 +1147,8 @@ u64 bpf_jit_alloc_exec_limit(void)
>
> void *bpf_jit_alloc_exec(unsigned long size)
> {
> - return vmalloc(size);
> + /* Memory is intended to be executable, reset the pointer tag. */
> + return kasan_reset_tag(vmalloc(size));
> }
>
> void bpf_jit_free_exec(void *addr)
> --
> 2.25.1

Hi Catalin,

I had to change this patch to fix an issue I discovered during
testing. Could you PTAL once again?

Thanks!