[RFC6 PATCH v6 00/21] ILP32 for ARM64

Catalin Marinas
2016-05-12T13:35:34Z

On Thu, May 12, 2016 at 03:20:00AM +0300, Yury Norov wrote:
> I debugged preadv02 and pwritev02 failures and found very weird bug.
> Test passes {iovec_base = 0xffffffff, iovec_len = 64} as one element
> of vector, and kernel reports successful read/write.
>
> There are 2 problems:
> 1. How kernel allows such address to be passed to fs subsystem;
> 2. How fs successes to read/write at non-mapped, and in fact non-user
> address.
>
> I don't know the answer on 2'nd question, and it might be something
> generic. But I investigated first problem.
>
> The problem is that compat_rw_copy_check_uvector() uses access_ok() to
> validate user address, and on arm64 it ends up with checking buffer
> end against current_thread_info()->addr_limit.
>
> current_thread_info()->addr_limit for ilp32, and most probably for
> aarch32 is equal to aarch64 one, and so adress_ok() doesn't fail.
> It happens because on thread creation we call flush_old_exec() to set
> addr_limit, and completely ignore compat mode there.

I assume accesses beyond this address would fault anyway but I haven't
checked the code paths.

> diff --git a/arch/arm64/include/asm/elf.h b/arch/arm64/include/asm/elf.h
> index 7a39683..6ba4952 100644
> --- a/arch/arm64/include/asm/elf.h
> +++ b/arch/arm64/include/asm/elf.h
> @@ -146,6 +146,7 @@ typedef struct user_fpsimd_state elf_fpregset_t;
> do { \
> clear_thread_flag(TIF_32BIT_AARCH64); \
> clear_thread_flag(TIF_32BIT); \
> + set_fs(TASK_SIZE_64); \
> } while (0)

See below.

> diff --git a/arch/arm64/include/asm/uaccess.h b/arch/arm64/include/asm/uaccess.h
> index 19cfdc5..3b0dd8d 100644
> --- a/arch/arm64/include/asm/uaccess.h
> +++ b/arch/arm64/include/asm/uaccess.h
> @@ -51,7 +51,7 @@
> #define KERNEL_DS (-1UL)
> #define get_ds() (KERNEL_DS)
>
> -#define USER_DS TASK_SIZE_64
> +#define USER_DS TASK_SIZE

I agree with this.

> #define get_fs() (current_thread_info()->addr_limit)
>
> static inline void set_fs(mm_segment_t fs)
> diff --git a/arch/arm64/kernel/binfmt_elf32.c b/arch/arm64/kernel/binfmt_elf32.c
> index 5487872..2e8d9f3 100644
> --- a/arch/arm64/kernel/binfmt_elf32.c
> +++ b/arch/arm64/kernel/binfmt_elf32.c
> @@ -12,6 +12,7 @@
> do { \
> clear_thread_flag(TIF_32BIT_AARCH64); \
> set_thread_flag(TIF_32BIT); \
> + set_fs(TASK_SIZE_32); \
> } while (0)
>
> #define COMPAT_ARCH_DLINFO
> diff --git a/arch/arm64/kernel/binfmt_ilp32.c b/arch/arm64/kernel/binfmt_ilp32.c
> index a934fd4..a8599c6 100644
> --- a/arch/arm64/kernel/binfmt_ilp32.c
> +++ b/arch/arm64/kernel/binfmt_ilp32.c
> @@ -59,6 +59,7 @@ static void cputime_to_compat_timeval(const cputime_t cputime,
> do { \
> set_thread_flag(TIF_32BIT_AARCH64); \
> clear_thread_flag(TIF_32BIT); \
> + set_fs(TASK_SIZE_32); \
> } while (0)

I don't think we need these two. AFAICT, flush_old_exec() takes care of
setting the USER_DS for the new thread.

--
Catalin