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Message-ID: <CAGXu5jKmTSb0YmNFDtr-jymrhy_s_PXUuvj3WKYffSU98gwyqg@mail.gmail.com>
Date: Wed, 9 Jul 2014 07:53:10 -0700
From: Kees Cook <keescook@...omium.org>
To: Cyrill Gorcunov <gorcunov@...il.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
LKML <linux-kernel@...r.kernel.org>, Tejun Heo <tj@...nel.org>,
Andrew Vagin <avagin@...nvz.org>,
"Eric W. Biederman" <ebiederm@...ssion.com>,
Serge Hallyn <serge.hallyn@...onical.com>,
Pavel Emelyanov <xemul@...allels.com>,
Vasiliy Kulikov <segoon@...nwall.com>,
KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
Michael Kerrisk <mtk.manpages@...il.com>
Subject: Re: [RFC 2/2] prctl: PR_SET_MM -- Introduce PR_SET_MM_MAP operation
On Wed, Jul 9, 2014 at 7:13 AM, Cyrill Gorcunov <gorcunov@...il.com> wrote:
> On Wed, Jul 09, 2014 at 02:13:36AM +0400, Cyrill Gorcunov wrote:
>>
>> Still the good news about all this members we modify -- they are used
>> for statistics mostly except brk/stack related members but they
>> are checked very carefully to not exceed the limits (if the
>> limits are set).
>>
>> > struct prctl_mm_map could do with a nice comment explaining its role in
>> > the world.
>>
>> ok, i'll update
>>
>> > I'm not seeing a coherent description of the proposed userspace
>> > interface. We'll eventually want to update the prctl manpage for this,
>> > so how about laying out all the needed details now, at patch review
>> > time so we can see what is proposed.
>>
>> Sure, I'll write more descriptive comment since original "It takes
>> a pointer of prctl_mm_map structure which carries all members to be
>> updated" is too short.
>
> Here is a way more descriptove changelog I hope. Please poke me if
> more details needed, or something should be improved/changed and
> etc.
> ---
> From: Cyrill Gorcunov <gorcunov@...nvz.org>
> Subject: prctl: PR_SET_MM -- Introduce PR_SET_MM_MAP operation
>
> During development of c/r we've noticed that in case if we need to
> support user namespaces we face a problem with capabilities in
> prctl(PR_SET_MM, ...) call, in particular once new user namespace
> is created capable(CAP_SYS_RESOURCE) no longer passes.
>
> A approach is to eliminate CAP_SYS_RESOURCE check but pass all
> new values in one bundle, which would allow the kernel to make
> more intensive test for sanity of values and same time allow us to
> support checkpoint/restore of user namespaces.
>
> Thus a new command PR_SET_MM_MAP introduced. It takes a pointer of
> prctl_mm_map structure which carries all the members to be updated.
>
> prctl(PR_SET_MM, PR_SET_MM_MAP, struct prctl_mm_map *, size)
>
> struct prctl_mm_map {
> __u64 start_code;
> __u64 end_code;
> __u64 start_data;
> __u64 end_data;
> __u64 start_brk;
> __u64 brk;
> __u64 start_stack;
> __u64 arg_start;
> __u64 arg_end;
> __u64 env_start;
> __u64 env_end;
> __u64 *auxv;
> __u32 auxv_size;
> __u32 exe_fd;
> };
>
> All members except @exe_fd correspond ones of struct mm_struct.
> To figure out which available values these members may take here
> are meanings of the members.
>
> - start_code, end_code: represent bounds of executable code area
> - start_data, end_data: represent bounds of data area
> - start_brk, brk: used to calculate bounds for brk() syscall
> - start_stack: used when accounting space needed for command
> line arguments, environment and shmat() syscall
> - arg_start, arg_end, env_start, env_end: represent memory area
> supplied for command line arguments and environment variables
> - auxv, auxv_size: carries auxiliary vector, Elf format specifics
> - exe_fd: file descriptor number for executable link (/proc/self/exe)
>
> Thus we apply the following requirements to the values
>
> 1) Any member except @auxv, @auxv_size, @exe_fd is rather an address
> in user space thus it must be laying inside [mmap_min_addr, mmap_max_addr)
> interval.
>
> 2) While @[start|end]_code and @[start|end]_data may point to an nonexisting
> VMAs (say a program maps own new .text and .data segments during execution)
> the rest of members should belong to VMA which must exist.
>
> 3) Addresses must be ordered, ie @start_ member must not be greater or
> equal to appropriate @end_ member.
>
> 4) As in regular Elf loading procedure we require that @start_brk and
> @brk be greater than @end_data.
>
> 5) If RLIMIT_DATA rlimit is set to non-infinity new values should not
> exceed existing limit. Same applies to RLIMIT_STACK.
>
> 6) Auxiliary vector size must not exceed existing one (which is
> predefined as AT_VECTOR_SIZE and depends on architecture).
>
> 7) File descriptor passed in @exe_file should be pointing
> to executable file (because we use existing prctl_set_mm_exe_file_locked
> helper it ensures that the file we are going to use as exe link has all
> required permission granted).
>
> Now about where these members are involved inside kernel code:
>
> - @start_code and @end_code are used in /proc/$pid/[stat|statm] output;
>
> - @start_data and @end_data are used in /proc/$pid/[stat|statm] output,
> also they are considered if there enough space for brk() syscall
> result if RLIMIT_DATA is set;
>
> - @start_brk shown in /proc/$pid/stat output and accounted in brk()
> syscall if RLIMIT_DATA is set; also this member is tested to
> find a symbolic name of mmap event for perf system (we choose
> if event is generated for "heap" area); one more aplication is
> selinux -- we test if a process has PROCESS__EXECHEAP permission
> if trying to make heap area being executable with mprotect() syscall;
>
> - @brk is a current value for brk() syscall which lays inside heap
> area, it's shown in /proc/$pid/stat. When syscall brk() succesfully
> provides new memory area to a user space upon brk() completion the
> mm::brk is updated to carry new value;
>
> Both @start_brk and @brk are actively used in /proc/$pid/maps
> and /proc/$pid/smaps output to find a symbolic name "heap" for
> VMA being scanned;
>
> - @start_stack is printed out in /proc/$pid/stat and used to
> find a symbolic name "stack" for task and threads in
> /proc/$pid/maps and /proc/$pid/smaps output, and as the same
> as with @start_brk -- perf system uses it for event naming.
> Also kernel treat this member as a start address of where
> to map vDSO pages and to check if there is enough space
> for shmat() syscall;
>
> - @arg_start, @arg_end, @env_start and @env_end are printed out
> in /proc/$pid/stat. Another access to the data these members
> represent is to read /proc/$pid/environ or /proc/$pid/cmdline.
> Any attempt to read these areas kernel tests with access_process_vm
> helper so a user must have enough rights for this action;
>
> - @auxv and @auxv_size may be read from /proc/$pid/auxv. Strictly
> speaking kernel doesn't care much about which exactly data is
> sitting there because it is solely for userspace;
>
> - @exe_fd is referred from /proc/$pid/exe and when generating
> coredump. We uses prctl_set_mm_exe_file_locked helper to update
> this member, so exe-file link modification remains one-shot
> action.
>
> Still note that updating exe-file link now doesn't require sys-resource
> capability anymore, after all there is no much profit in preventing setup
> own file link (there are a number of ways to execute own code -- ptrace,
> ld-preload, so that the only reliable way to find which exactly code
> is executed is to inspect running program memory).
>
> I believe the old interface should be deprecated and ripped off
> in a couple of kernel releases if no one against.
>
> To test if new interface is implemented in the kernel one
> can pass PR_SET_MM_MAP_SIZE opcode and the kernel returns
> the size of currently supported struct prctl_mm_map.
>
> Signed-off-by: Cyrill Gorcunov <gorcunov@...nvz.org>
> Cc: Kees Cook <keescook@...omium.org>
> Cc: Tejun Heo <tj@...nel.org>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Andrew Vagin <avagin@...nvz.org>
> Cc: Eric W. Biederman <ebiederm@...ssion.com>
> Cc: H. Peter Anvin <hpa@...or.com>
> Cc: Serge Hallyn <serge.hallyn@...onical.com>
> Cc: Pavel Emelyanov <xemul@...allels.com>
> Cc: Vasiliy Kulikov <segoon@...nwall.com>
> Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
> Cc: Michael Kerrisk <mtk.manpages@...il.com>
> ---
> include/uapi/linux/prctl.h | 25 +++++
> kernel/sys.c | 192 ++++++++++++++++++++++++++++++++++++++++++++-
> 2 files changed, 216 insertions(+), 1 deletion(-)
>
> Index: linux-2.6.git/include/uapi/linux/prctl.h
> ===================================================================
> --- linux-2.6.git.orig/include/uapi/linux/prctl.h
> +++ linux-2.6.git/include/uapi/linux/prctl.h
> @@ -119,6 +119,31 @@
> # define PR_SET_MM_ENV_END 11
> # define PR_SET_MM_AUXV 12
> # define PR_SET_MM_EXE_FILE 13
> +# define PR_SET_MM_MAP 14
> +# define PR_SET_MM_MAP_SIZE 15
> +
> +/*
> + * This structure provides new memory descriptor
> + * map which mostly modifies /proc/pid/stat[m]
> + * output for a task. This mostly done in a
> + * sake of checkpoint/restore functionality.
> + */
> +struct prctl_mm_map {
> + __u64 start_code; /* code section bounds */
> + __u64 end_code;
> + __u64 start_data; /* data section bounds */
> + __u64 end_data;
> + __u64 start_brk; /* heap for brk() syscall */
> + __u64 brk;
> + __u64 start_stack; /* stack starts at */
> + __u64 arg_start; /* command line arguments bounds */
> + __u64 arg_end;
> + __u64 env_start; /* environment variables bounds */
> + __u64 env_end;
> + __u64 *auxv; /* auxiliary vector */
> + __u32 auxv_size; /* vector size */
> + __u32 exe_fd; /* /proc/$pid/exe link file */
> +};
>
> /*
> * Set specific pid that is allowed to ptrace the current task.
> Index: linux-2.6.git/kernel/sys.c
> ===================================================================
> --- linux-2.6.git.orig/kernel/sys.c
> +++ linux-2.6.git/kernel/sys.c
> @@ -1687,6 +1687,189 @@ exit:
> return err;
> }
>
> +#ifdef CONFIG_CHECKPOINT_RESTORE
> +/*
> + * WARNING: we don't require any capability here so be very careful
> + * in what is allowed for modification from userspace.
> + */
> +static int validate_prctl_map_locked(struct prctl_mm_map *prctl_map)
> +{
> + unsigned long mmap_max_addr = TASK_SIZE;
> + struct mm_struct *mm = current->mm;
> + struct vm_area_struct *stack_vma;
> + unsigned long rlim;
> + int error = 0;
> +
> + /*
> + * Make sure the members are not somewhere outside
> + * of allowed address space.
> + */
> +#define __prctl_check_addr_space(__map, __member) \
> + ({ \
> + int __rc; \
> + if ((unsigned long)__map->__member < mmap_max_addr && \
> + (unsigned long)__map->__member >= mmap_min_addr) \
> + __rc = 0; \
> + else \
> + __rc = -EINVAL; \
> + __rc; \
> + })
> +
> + error |= __prctl_check_addr_space(prctl_map, start_code);
> + error |= __prctl_check_addr_space(prctl_map, end_code);
> + error |= __prctl_check_addr_space(prctl_map, start_data);
> + error |= __prctl_check_addr_space(prctl_map, end_data);
> + error |= __prctl_check_addr_space(prctl_map, start_stack);
> + error |= __prctl_check_addr_space(prctl_map, start_brk);
> + error |= __prctl_check_addr_space(prctl_map, brk);
> + error |= __prctl_check_addr_space(prctl_map, arg_start);
> + error |= __prctl_check_addr_space(prctl_map, arg_end);
> + error |= __prctl_check_addr_space(prctl_map, env_start);
> + error |= __prctl_check_addr_space(prctl_map, env_end);
> + if (error)
> + goto out;
> +#undef __prctl_check_addr_space
> +
> + /*
> + * Stack, brk, command line arguments and environment must exist.
> + */
> + stack_vma = find_vma(mm, (unsigned long)prctl_map->start_stack);
> + if (!stack_vma) {
> + error = -EINVAL;
> + goto out;
> + }
> +#define __prctl_check_vma(mm, addr) find_vma(mm, (unsigned long)addr) ? 0 : -EINVAL
> + error |= __prctl_check_vma(mm, prctl_map->start_brk);
> + error |= __prctl_check_vma(mm, prctl_map->brk);
> + error |= __prctl_check_vma(mm, prctl_map->arg_start);
> + error |= __prctl_check_vma(mm, prctl_map->arg_end);
> + error |= __prctl_check_vma(mm, prctl_map->env_start);
> + error |= __prctl_check_vma(mm, prctl_map->env_end);
> + if (error)
> + goto out;
> +#undef __prctl_check_vma
> +
> + /*
> + * Make sure the pairs are ordered.
> + */
> +#define __prctl_check_order(__map, __m1, __m2) \
> + (unsigned long)__map->__m2 <= (unsigned long)__map->__m1
> + if (__prctl_check_order(prctl_map, start_code, end_code) ||
> + __prctl_check_order(prctl_map, start_data, end_data) ||
> + __prctl_check_order(prctl_map, arg_start, arg_end) ||
> + __prctl_check_order(prctl_map, env_start, env_end))
> + goto out;
> +#undef __prctl_check_order
This approach seems like a good solution given the security concerns
with the earlier approach. I'm still pondering the implications, but
as a minor style note, these macros are locally defined, but also all
take at least a single identical argument in every usage. I would
think it might be easier to read if they just used what they needed to
directly.
#define __prctl_check_addr_space(__member) \
((unsigned long)prctl_map->__member < mmap_max_addr && \
(unsigned long)prctl_map->__member >= mmap_min_addr) ? 0 : -EINVAL
#define __prctl_check_vma(__member) \
find_vma(mm, (unsigned long)prctl_map->__member) ? 0 : -EINVAL
Also, why change the symantics of the final macro? Seems like that one
can use the same "error |=" style:
#define __prctl_check_order(__m1, __m2) \
prctl_map->__m1 < prctl_map->__m2 ? 0 : -EINVAL
-Kees
--
Kees Cook
Chrome OS Security
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