On Mon, Jun 8, 2015 at 8:39 PM, Calvin Owens <calvinow...@fb.com> wrote:
> Currently, /proc/<pid>/map_files/ is restricted to CAP_SYS_ADMIN, and
> is only exposed if CONFIG_CHECKPOINT_RESTORE is set.
>
> This interface very useful because it allows userspace to stat()
> deleted files that are still mapped by some process, which enables a
> much quicker and more accurate answer to the question "How much disk
> space is being consumed by files that are deleted but still mapped?"
> than is currently possible.
>
> This patch moves map_files/ out from behind CONFIG_CHECKPOINT_RESTORE,
> and adjusts the permissions enforced on it as follows:
>
> * proc_map_files_lookup()
> * proc_map_files_readdir()
> * map_files_d_revalidate()
>
> Remove the CAP_SYS_ADMIN restriction, leaving only the current
> restriction requiring PTRACE_MODE_READ.
>
> In earlier versions of this patch, I changed the ptrace checks
> in the functions above to enforce MODE_ATTACH instead of
> MODE_READ. That was an oversight: all the information exposed
> by the above three functions is already available with
> MODE_READ from /proc/PID/maps. I was only being asked to
> strengthen the protection around functionality provided by
> follow_link(), not the above.
>
> So, I've left the checks for MODE_READ as-is, since AFAICS all
> objections raised so far are addressed by the new CAP_SYS_ADMIN
> check in follow_link(), explained below.
>
> * proc_map_files_follow_link()
>
> This stub has been added, and requires that the user have
> CAP_SYS_ADMIN in order to follow the links in map_files/,
> since there was concern on LKML both about the potential for
> bypassing permissions on ancestor directories in the path to
> files pointed to, and about what happens with more exotic
> memory mappings created by some drivers (ie dma-buf).
>
> Cc: Andrew Morton <a...@linux-foundation.org>
> Cc: Andy Lutomirski <l...@amacapital.net>
> Cc: Cyrill Gorcunov <gorcu...@openvz.org>
> Cc: Joe Perches <j...@perches.com>
> Cc: Kees Cook <keesc...@chromium.org>
> Cc: Kirill A. Shutemov <kir...@shutemov.name>
> Signed-off-by: Calvin Owens <calvinow...@fb.com>
> ---
> Changes in v6: Require CAP_SYS_ADMIN for follow_link(). Leave other
> PTRACE_MODE_READ checks as-is, since CAP_SYS_ADMIN
> alone addresses all concerns raised AFAICS.
>
> Changes in v5: s/dentry->d_inode/d_inode(dentry)/g
>
> Changes in v4: Return -ESRCH from follow_link() when get_proc_task()
> returns NULL.
>
> Changes in v3: Changed permission checks to use PTRACE_MODE_ATTACH
> instead of PTRACE_MODE_READ, and added a stub to
> enforce MODE_ATTACH on follow_link() as well.
>
> Changes in v2: Removed the follow_link() stub that returned -EPERM if
> the caller didn't have CAP_SYS_ADMIN, since the caller
> in my chroot() scenario gets -EACCES anyway.
>
> fs/proc/base.c | 42 +++++++++++++++++++++++-------------------
> 1 file changed, 23 insertions(+), 19 deletions(-)
>
> diff --git a/fs/proc/base.c b/fs/proc/base.c
> index 093ca14..0270191 100644
> --- a/fs/proc/base.c
> +++ b/fs/proc/base.c
> @@ -1641,8 +1641,6 @@ end_instantiate:
> return dir_emit(ctx, name, len, 1, DT_UNKNOWN);
> }
>
> -#ifdef CONFIG_CHECKPOINT_RESTORE
> -
> /*
> * dname_to_vma_addr - maps a dentry name into two unsigned longs
> * which represent vma start and end addresses.
> @@ -1669,11 +1667,6 @@ static int map_files_d_revalidate(struct dentry
> *dentry, unsigned int flags)
> if (flags & LOOKUP_RCU)
> return -ECHILD;
>
> - if (!capable(CAP_SYS_ADMIN)) {
> - status = -EPERM;
> - goto out_notask;
> - }
> -
> inode = d_inode(dentry);
> task = get_proc_task(inode);
> if (!task)
> @@ -1762,6 +1755,28 @@ struct map_files_info {
> unsigned char name[4*sizeof(long)+2]; /* max: %lx-%lx\0 */
> };
>
> +/*
> + * Only allow CAP_SYS_ADMIN to follow the links, due to concerns about how
> the
> + * symlinks may be used to bypass permissions on ancestor directories in the
> + * path to the file in question.
> + */
Cool, I think this looks good. Thanks!
Reviewed-by: Kees Cook <keesc...@chromium.org>
-Kees
> +static void *proc_map_files_follow_link(struct dentry *dentry, struct
> nameidata *nd)
> +{
> + if (!capable(CAP_SYS_ADMIN))
> + return ERR_PTR(-EPERM);
> +
> + return proc_pid_follow_link(dentry, nd);
> +}
> +
> +/*
> + * Identical to proc_pid_link_inode_operations except for follow_link()
> + */
> +static const struct inode_operations proc_map_files_link_inode_operations = {
> + .readlink = proc_pid_readlink,
> + .follow_link = proc_map_files_follow_link,
> + .setattr = proc_setattr,
> +};
> +
> static int
> proc_map_files_instantiate(struct inode *dir, struct dentry *dentry,
> struct task_struct *task, const void *ptr)
> @@ -1777,7 +1792,7 @@ proc_map_files_instantiate(struct inode *dir, struct
> dentry *dentry,
> ei = PROC_I(inode);
> ei->op.proc_get_link = proc_map_files_get_link;
>
> - inode->i_op = &proc_pid_link_inode_operations;
> + inode->i_op = &proc_map_files_link_inode_operations;
> inode->i_size = 64;
> inode->i_mode = S_IFLNK;
>
> @@ -1801,10 +1816,6 @@ static struct dentry *proc_map_files_lookup(struct
> inode *dir,
> int result;
> struct mm_struct *mm;
>
> - result = -EPERM;
> - if (!capable(CAP_SYS_ADMIN))
> - goto out;
> -
> result = -ENOENT;
> task = get_proc_task(dir);
> if (!task)
> @@ -1858,10 +1869,6 @@ proc_map_files_readdir(struct file *file, struct
> dir_context *ctx)
> struct map_files_info *p;
> int ret;
>
> - ret = -EPERM;
> - if (!capable(CAP_SYS_ADMIN))
> - goto out;
> -
> ret = -ENOENT;
> task = get_proc_task(file_inode(file));
> if (!task)
> @@ -2050,7 +2057,6 @@ static const struct file_operations
> proc_timers_operations = {
> .llseek = seq_lseek,
> .release = seq_release_private,
> };
> -#endif /* CONFIG_CHECKPOINT_RESTORE */
>
> static int proc_pident_instantiate(struct inode *dir,
> struct dentry *dentry, struct task_struct *task, const void *ptr)
> @@ -2549,9 +2555,7 @@ static const struct inode_operations
> proc_task_inode_operations;
> static const struct pid_entry tgid_base_stuff[] = {
> DIR("task", S_IRUGO|S_IXUGO, proc_task_inode_operations,
> proc_task_operations),
> DIR("fd", S_IRUSR|S_IXUSR, proc_fd_inode_operations,
> proc_fd_operations),
> -#ifdef CONFIG_CHECKPOINT_RESTORE
> DIR("map_files", S_IRUSR|S_IXUSR, proc_map_files_inode_operations,
> proc_map_files_operations),
> -#endif
> DIR("fdinfo", S_IRUSR|S_IXUSR, proc_fdinfo_inode_operations,
> proc_fdinfo_operations),
> DIR("ns", S_IRUSR|S_IXUGO, proc_ns_dir_inode_operations,
> proc_ns_dir_operations),
> #ifdef CONFIG_NET
> --
> 1.8.1
>
--
Kees Cook
Chrome OS Security
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