Messages in this thread | | | From | Djalal Harouni <> | Subject | [PATCH 0/12] procfs: protect /proc/<pid>/* files with file->f_cred | Date | Wed, 25 Sep 2013 21:14:33 +0100 |
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/proc/<pid>/* entries varies at runtime, appropriate permission checks need to happen during each system call.
Currently some of these sensitive entries are protected by performing the ptrace_may_access() check. However even with that the /proc file descriptors can be passed to a more privileged process (e.g. a suid-exec) which will pass the classic ptrace_may_access() check. In general the ->open() call will be issued by an unprivileged process while the ->read(),->write() calls by a more privileged one.
Example of these files are: /proc/*/syscall, /proc/*/stack etc.
And any open(/proc/self/*) then suid-exec to read()/write() /proc/self/*
These files are protected during read() by the ptrace_may_access(), however the file descriptor can be passed to a suid-exec which can be used to read data and bypass ASLR. Of course this was discussed several times on LKML.
Solution: Here we propose a clean solution that uses available mechanisms. No additions, nor new structs/memory allocation...
After a discussion about some /proc/<pid>/* file permissions [1], Eric W. Biederman proposed to use the file->f_cred to check if current's cred have changed [2], actually he said that Linus was looking on using the file->f_cred to implement a similar thing! But run in problems with Chromes sandbox ? a link please ?
So here are my experiments: The idea is to track the cred of current. If the cred change between ->open() and read()/write() this means that current has lost or gained some X privileges. If so, in addition to the classic ptrace_may_access() check, perform a second check using the file's opener cred. This means, if an unprivileged process that tries to use a privileged one (e.g. suid-exec) to read privileged files will get caught. The original process that opened the file does not have the appropriate permissions to read()/write() the target /proc/<pid>/* entry.
This second check is done of course during read(),write()...
Advantages of the proposed solution: * It uses available mechanisms: file->f_cred which is a const cred that reference the cred of the opener.
* The file->f_cred can be easily referenced and it will live until fput()
* Does not pin or grab any counter on any extra struct.
* It allows to pass file descriptors under certain conditions: (1) current at open time may have enough permissions (2) current does a suid-exec or change its ruid/euid (new cred) (3) current or suid-exec tries to read from the task /proc entry Allow the ->read() only if the file's opener cred at (1) have enough permissions on *this* task /proc entry during *this* ->read() moment. Otherwise fail.
IOW allow it if the opener does not need the help of a suid-exec to read/write data.
Disadvantage: * Currently only /proc/*/{stack,syscall,stat,personality} are handled If the solution is accepted I'll continue with other files, taking care to not break userspace. All the facilities are provided. * Your feedback
The following series tries to implement what I describe.
1) Add the proc_same_open_cred() helper This function will check if the file's opener cred matches the current cred. The match concerns the cred that are used in the ptrace_may_access() check to allow /proc task access. These cred are: uid,gid and cap_permitted.
2) Add the proc_allow_access() helper Check if the file's opener had enough permissions to access the target process.
3) Make seq_file struct able to access the file's opener cred. Since seq_file struct is used by procfs entries, embed a pointer to the file->f_cred into the seq_file struct.
4) Add permission checks on the file's opener of /proc/*/stack
5) Add permission checks on the file's opener of /proc/*/personality
6) Add permission checks on the file's opener of /proc/*/stat
7) Improve permission checks on /proc/*/syscall
8) Finally the last patch is user_ns and seq_file cleaning.
Thanks!
[1] https://lkml.org/lkml/2013/8/26/354 [2] https://lkml.org/lkml/2013/8/31/209
Djalal Harouni (12): procfs: add proc_same_open_cred() to check if the cred have changed procfs: add proc_allow_access() to check if file's opener may access task procfs: Document the proposed solution to protect procfs entries seq_file: Make seq_file able to access the file's opener cred seq_file: set the seq_file->f_cred during seq_open() procfs: make /proc/*/stack 0400 procfs: add permission checks on the file's opener of /proc/*/stack procfs: add permission checks on the file's opener of /proc/*/personality procfs: add permission checks on the file's opener of /proc/*/stat procfs: move PROC_BLOCK_SIZE declaration up to make it visible procfs: improve permission checks on /proc/*/syscall user_ns: seq_file: use the user_ns that is embedded in the f_cred struct
fs/proc/array.c | 14 ++++++- fs/proc/base.c | 233 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++---------- fs/proc/internal.h | 3 ++ fs/seq_file.c | 4 +- include/linux/seq_file.h | 13 +++++-- 5 files changed, 238 insertions(+), 29 deletions(-)
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