linux-kernel - Re: [RFC6 PATCH v6 00/21] ILP32 for ARM64

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Date:	Thu, 12 May 2016 14:35:34 +0100
From:	Catalin Marinas <catalin.marinas@....com>
To:	Yury Norov <ynorov@...iumnetworks.com>
Cc:	arnd@...db.de, linux-arm-kernel@...ts.infradead.org,
	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
	linux-s390@...r.kernel.org, pinskia@...il.com,
	Prasun.Kapoor@...iumnetworks.com, heiko.carstens@...ibm.com,
	linux-doc@...r.kernel.org, Nathan_Lynch@...tor.com, agraf@...e.de,
	klimov.linux@...il.com, broonie@...nel.org,
	bamvor.zhangjian@...wei.com, schwab@...e.de,
	schwidefsky@...ibm.com, joseph@...esourcery.com,
	christoph.muellner@...obroma-systems.com
Subject: Re: [RFC6 PATCH v6 00/21] ILP32 for ARM64

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