Re: [patch 4/4] prctl: PR_SET_MM -- Introduce PR_SET_MM_MAP operation, v3

[Serge E. Hallyn]

Quoting Cyrill Gorcunov (gorcunov@xxxxxxxxxx):
> 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). Still we require
> the caller to be at least user-namespace root user.
> 
> 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.
> 
> v2:
>  - compact macros (by keescook@)
>  - wrap new code with CONFIG_ (by akpm@)
> 
> v3 (by jln@):
>  - use __prctl_check_order for brk and start_brk
>  - use may_adjust_brk helper
>  - make sure that only root can update @exe_fd link
> 
> Signed-off-by: Cyrill Gorcunov <gorcunov@xxxxxxxxxx>
> Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> Cc: Tejun Heo <tj@xxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Andrew Vagin <avagin@xxxxxxxxxx>
> Cc: Eric W. Biederman <ebiederm@xxxxxxxxxxxx>
> Cc: H. Peter Anvin <hpa@xxxxxxxxx>
> Cc: Serge Hallyn <serge.hallyn@xxxxxxxxxxxxx>

Going mainly by your description, as this is not my forté.
All looks kosher to me.

Acked-by: Serge Hallyn <serge.hallyn@xxxxxxxxxxxxx>

> Cc: Pavel Emelyanov <xemul@xxxxxxxxxxxxx>
> Cc: Vasiliy Kulikov <segoon@xxxxxxxxxxxx>
> Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
> Cc: Michael Kerrisk <mtk.manpages@xxxxxxxxx>
> Cc: Julien Tinnes <jln@xxxxxxxxxx>
> ---
>  include/uapi/linux/prctl.h |   25 +++++
>  kernel/sys.c               |  211 ++++++++++++++++++++++++++++++++++++++++++++-
>  2 files changed, 235 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 +1687,208 @@ 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;
> +	int error = 0;
> +
> +	/*
> +	 * Make sure the members are not somewhere outside
> +	 * of allowed address space.
> +	 */
> +#define __prctl_check_addr_space(__member)					\
> +	({									\
> +		int __rc;							\
> +		if ((unsigned long)prctl_map->__member < mmap_max_addr &&	\
> +		    (unsigned long)prctl_map->__member >= mmap_min_addr)	\
> +			__rc = 0;						\
> +		else								\
> +			__rc = -EINVAL;						\
> +		__rc;								\
> +	})
> +	error |= __prctl_check_addr_space(start_code);
> +	error |= __prctl_check_addr_space(end_code);
> +	error |= __prctl_check_addr_space(start_data);
> +	error |= __prctl_check_addr_space(end_data);
> +	error |= __prctl_check_addr_space(start_stack);
> +	error |= __prctl_check_addr_space(start_brk);
> +	error |= __prctl_check_addr_space(brk);
> +	error |= __prctl_check_addr_space(arg_start);
> +	error |= __prctl_check_addr_space(arg_end);
> +	error |= __prctl_check_addr_space(env_start);
> +	error |= __prctl_check_addr_space(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(__member)						\
> +	find_vma(mm, (unsigned long)prctl_map->__member) ? 0 : -EINVAL
> +	error |= __prctl_check_vma(start_brk);
> +	error |= __prctl_check_vma(brk);
> +	error |= __prctl_check_vma(arg_start);
> +	error |= __prctl_check_vma(arg_end);
> +	error |= __prctl_check_vma(env_start);
> +	error |= __prctl_check_vma(env_end);
> +	if (error)
> +		goto out;
> +#undef __prctl_check_vma
> +
> +	/*
> +	 * Make sure the pairs are ordered.
> +	 */
> +#define __prctl_check_order(__m1, __op, __m2)					\
> +	((unsigned long)prctl_map->__m1 __op					\
> +	 (unsigned long)prctl_map->__m2) ? 0 : -EINVAL
> +	error |= __prctl_check_order(start_code, <, end_code);
> +	error |= __prctl_check_order(start_data, <, end_data);
> +	error |= __prctl_check_order(start_brk, <=, brk);
> +	error |= __prctl_check_order(arg_start, <=, arg_end);
> +	error |= __prctl_check_order(env_start, <=, env_end);
> +	if (error)
> +		goto out;
> +#undef __prctl_check_order
> +
> +	error = -EINVAL;
> +
> +	/*
> +	 * @brk should be after @end_data in traditional maps.
> +	 */
> +	if (prctl_map->start_brk <= prctl_map->end_data ||
> +	    prctl_map->brk <= prctl_map->end_data)
> +		goto out;
> +
> +	/*
> +	 * Neither we should allow to override limits if they set.
> +	 */
> +	if (check_data_rlimit(rlimit(RLIMIT_DATA), prctl_map->brk,
> +			      prctl_map->start_brk, prctl_map->end_data,
> +			      prctl_map->start_data))
> +			goto out;
> +
> +#ifdef CONFIG_STACK_GROWSUP
> +	if (check_data_rlimit(rlimit(RLIMIT_STACK),
> +			      stack_vma->vm_end,
> +			      prctl_map->start_stack, 0, 0))
> +#else
> +	if (check_data_rlimit(rlimit(RLIMIT_STACK),
> +			      prctl_map->start_stack,
> +			      stack_vma->vm_start, 0, 0))
> +#endif
> +		goto out;
> +
> +	/*
> +	 * Someone is trying to cheat the auxv vector.
> +	 */
> +	if (prctl_map->auxv_size) {
> +		if (!prctl_map->auxv ||
> +		    prctl_map->auxv_size > sizeof(mm->saved_auxv))
> +			goto out;
> +	}
> +
> +	/*
> +	 * Finally, make sure the caller has the rights to
> +	 * change /proc/pid/exe link: only local root should
> +	 * be allowed to.
> +	 */
> +	if (prctl_map->exe_fd != (u32)-1) {
> +		struct user_namespace *ns = current_user_ns();
> +		const struct cred *cred = current_cred();
> +
> +		if (!uid_eq(cred->uid, make_kuid(ns, 0)) ||
> +		    !gid_eq(cred->gid, make_kgid(ns, 0)))
> +			goto out;
> +	}
> +
> +	error = 0;
> +out:
> +	return error;
> +}
> +
> +static int prctl_set_mm_map(int opt, const void __user *addr, unsigned long data_size)
> +{
> +	struct prctl_mm_map prctl_map = { .exe_fd = (u32)-1, };
> +	unsigned long user_auxv[AT_VECTOR_SIZE];
> +	struct mm_struct *mm = current->mm;
> +	int error = -EINVAL;
> +
> +	BUILD_BUG_ON(sizeof(user_auxv) != sizeof(mm->saved_auxv));
> +
> +	if (opt == PR_SET_MM_MAP_SIZE)
> +		return put_user((unsigned int)sizeof(prctl_map),
> +				(unsigned int __user *)addr);
> +
> +	if (data_size != sizeof(prctl_map))
> +		return -EINVAL;
> +
> +	if (copy_from_user(&prctl_map, addr, sizeof(prctl_map)))
> +		return -EFAULT;
> +
> +	down_read(&mm->mmap_sem);
> +
> +	if (validate_prctl_map_locked(&prctl_map))
> +		goto out;
> +
> +	if (prctl_map.auxv_size) {
> +		up_read(&mm->mmap_sem);
> +		memset(user_auxv, 0, sizeof(user_auxv));
> +		error = copy_from_user(user_auxv,
> +				       (const void __user *)prctl_map.auxv,
> +				       prctl_map.auxv_size);
> +		down_read(&mm->mmap_sem);
> +		if (error)
> +			goto out;
> +	}
> +
> +	if (prctl_map.exe_fd != (u32)-1) {
> +		error = prctl_set_mm_exe_file_locked(mm, prctl_map.exe_fd);
> +		if (error)
> +			goto out;
> +	}
> +
> +	if (prctl_map.auxv_size) {
> +		/* Last entry must be AT_NULL as specification requires */
> +		user_auxv[AT_VECTOR_SIZE - 2] = AT_NULL;
> +		user_auxv[AT_VECTOR_SIZE - 1] = AT_NULL;
> +
> +		task_lock(current);
> +		memcpy(mm->saved_auxv, user_auxv, sizeof(user_auxv));
> +		task_unlock(current);
> +	}
> +
> +	mm->start_code	= prctl_map.start_code;
> +	mm->end_code	= prctl_map.end_code;
> +	mm->start_data	= prctl_map.start_data;
> +	mm->end_data	= prctl_map.end_data;
> +	mm->start_brk	= prctl_map.start_brk;
> +	mm->brk		= prctl_map.brk;
> +	mm->start_stack	= prctl_map.start_stack;
> +	mm->arg_start	= prctl_map.arg_start;
> +	mm->arg_end	= prctl_map.arg_end;
> +	mm->env_start	= prctl_map.env_start;
> +	mm->env_end	= prctl_map.env_end;
> +
> +	error = 0;
> +out:
> +	up_read(&mm->mmap_sem);
> +	return error;
> +}
> +#endif /* CONFIG_CHECKPOINT_RESTORE */
> +
>  static int prctl_set_mm(int opt, unsigned long addr,
>  			unsigned long arg4, unsigned long arg5)
>  {
> @@ -1694,9 +1896,16 @@ static int prctl_set_mm(int opt, unsigne
>  	struct vm_area_struct *vma;
>  	int error;
>  
> -	if (arg5 || (arg4 && opt != PR_SET_MM_AUXV))
> +	if (arg5 || (arg4 && (opt != PR_SET_MM_AUXV &&
> +			      opt != PR_SET_MM_MAP &&
> +			      opt != PR_SET_MM_MAP_SIZE)))
>  		return -EINVAL;
>  
> +#ifdef CONFIG_CHECKPOINT_RESTORE
> +	if (opt == PR_SET_MM_MAP || opt == PR_SET_MM_MAP_SIZE)
> +		return prctl_set_mm_map(opt, (const void __user *)addr, arg4);
> +#endif
> +
>  	if (!capable(CAP_SYS_RESOURCE))
>  		return -EPERM;
>  
> 
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