Messages in this thread | | | Date | Thu, 22 Jan 2015 13:00:25 -0800 | From | Calvin Owens <> | Subject | Re: [RFC][PATCH] procfs: Always expose /proc/<pid>/map_files/ and make it readable |
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On Thursday 01/22 at 13:02 +0200, Kirill A. Shutemov wrote: > On Wed, Jan 21, 2015 at 06:45:54PM -0800, Calvin Owens wrote: > > Currently, /proc/<pid>/map_files/ is restricted to CAP_SYS_ADMIN, and > > is only exposed if CONFIG_CHECKPOINT_RESTORE is set. This interface > > is very useful for enumerating the files mapped into a process when > > the more verbose information in /proc/<pid>/maps is not needed. > > > > This patch moves the folder out from behind CHECKPOINT_RESTORE, and > > removes the CAP_SYS_ADMIN restrictions. To avoid exposing files to > > processes for whom they may not be visible, a follow_link() stub is > > added to the inode_operations struct attached to the symlinks that > > prevent them from being followed without CAP_SYS_ADMIN. > > > > Signed-off-by: Calvin Owens <calvinowens@fb.com> > > --- > > 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 3f3d7ae..7d48003 100644 > > --- a/fs/proc/base.c > > +++ b/fs/proc/base.c > > @@ -1632,8 +1632,6 @@ 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. > > @@ -1660,11 +1658,6 @@ 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 = dentry->d_inode; > > task = get_proc_task(inode); > > if (!task) > > @@ -1753,6 +1746,28 @@ struct map_files_info { > > unsigned char name[4*sizeof(long)+2]; /* max: %lx-%lx\0 */ > > }; > > > > +/* > > + * Allowing any user to follow the symlinks in /proc/<pid>/map_files/ could > > + * allow processes to access files that should not be visible to them, so > > + * restrict follow_link() to CAP_SYS_ADMIN for these files. > > + */ > > +static void *proc_map_files_follow_link(struct dentry *d, struct nameidata *n) > > +{ > > + if (!capable(CAP_SYS_ADMIN)) > > + return ERR_PTR(-EPERM); > > + > > + return proc_pid_follow_link(d, n); > > +} > > I have thought a bit more about this and not sure it's reasonable to > limit it to CAP_SYS_ADMIN. What scenario are we protecting from? > > Initially, I thought about something like this: privileged process opens a > file, map part of it, closes the file and drop privileges with hope to > limit further access to mapped window of the file. But I don't see what > would stop the unprivileged process from accessing rest of the file using > mremap(2). And if a process can do this, anybody who can ptrace(2) the > process can do this. > > Am I missing something?
The specific case I was thinking of is a process in a chroot with a mounted /proc inside of it: if a process inside the chroot has the same UID as a process outside of it, the chroot'ed process could follow the symlinks in map_files/ and poke files it can't actually see, right?
I don't personally care about that use case, but it seemed like something that might surprise somebody.
Calvin
> -- > Kirill A. Shutemov
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