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SubjectRe: [PATCH v4] fs: make dumpable=2 only write to a pipe
Kees Cook <keescook@chromium.org> writes:

> When the suid_dumpable sysctl is set to "2", and there is no core
> dump pipe defined in the core_pattern sysctl, a local user can cause
> core files to be written to root-writable directories, potentially
> with user-controlled content. This means an admin can unknowningly
> reintroduce a variation of CVE-2006-2451, allowing local users to gain
> root privileges.

Is there a security exploit possible if a path is defined in the
core_pattern?

From the description of the problem so far I don't think there is.
Requiring a program when a simple path will do seems excessive.

Eric


> $ cat /proc/sys/fs/suid_dumpable
> 2
> $ cat /proc/sys/kernel/core_pattern
> core
> $ ulimit -c unlimited
> $ cd /
> $ ls -l core
> ls: cannot access core: No such file or directory
> $ touch core
> touch: cannot touch `core': Permission denied
> $ OHAI="evil-string-here" ping localhost >/dev/null 2>&1 &
> $ pid=$!
> $ sleep 1
> $ kill -SEGV $pid
> $ ls -l core
> -rw------- 1 root kees 458752 Jun 21 11:35 core
> $ sudo strings core | grep evil
> OHAI=evil-string-here
>
> While cron has been fixed to abort reading a file when there is any
> parse error, there are still other sensitive directories that will read
> any file present and skip unparsable lines.
>
> Instead of introducing a suid_dumpable=3 mode and breaking all users
> of mode 2, this only disables the unsafe portion of mode 2 (writing to
> disk). Most users of mode 2 (e.g. Chrome OS) already use a core dump
> pipe handler, so this change will not break them. For the situations
> where a pipe handler is not defined but mode 2 is still active, crash
> dumps will no longer be written to disk, and will instead trigger a
> printk yelling about the lack of the pipe handler.
>
> Signed-off-by: Kees Cook <keescook@chromium.org>
> ---
> v4:
> - drop mode 3 in favor of only half-breaking mode 2, requested by
> Andrew Morton.
> - yell loudly when encountering a mode 2 disk dump, suggested by
> Andrew Morton.
> v3:
> - use proper sysctl _conv function, fix commit description, suggested by
> Eric W. Biederman.
> v2:
> - switch to mode 3, remove mode 2, suggested by Alan Cox.
> ---
> Documentation/sysctl/fs.txt | 18 +++++++++++-------
> fs/exec.c | 37 ++++++++++++++++---------------------
> 2 files changed, 27 insertions(+), 28 deletions(-)
>
> diff --git a/Documentation/sysctl/fs.txt b/Documentation/sysctl/fs.txt
> index 13d6166..d20e2ea 100644
> --- a/Documentation/sysctl/fs.txt
> +++ b/Documentation/sysctl/fs.txt
> @@ -163,16 +163,20 @@ This value can be used to query and set the core dump mode for setuid
> or otherwise protected/tainted binaries. The modes are
>
> 0 - (default) - traditional behaviour. Any process which has changed
> - privilege levels or is execute only will not be dumped
> + privilege levels or is execute only will not be dumped.
> 1 - (debug) - all processes dump core when possible. The core dump is
> owned by the current user and no security is applied. This is
> intended for system debugging situations only. Ptrace is unchecked.
> -2 - (suidsafe) - any binary which normally would not be dumped is dumped
> - readable by root only. This allows the end user to remove
> - such a dump but not access it directly. For security reasons
> - core dumps in this mode will not overwrite one another or
> - other files. This mode is appropriate when administrators are
> - attempting to debug problems in a normal environment.
> + This is insecure as it allows regular users to examine the memory
> + contents of privileged processes.
> +2 - (pipeonly) - any binary which normally would not be dumped is dumped
> + anyway, but only if a core dump pipe handler is defined (see the
> + "core_pattern" kernel sysctl). This mode is appropriate when
> + administrators are attempting to debug problems in a normal
> + environment, and have a core dump pipe handler that knows to
> + treat privileged core dumps with care. If a core dump happens
> + without a pipe handler, a message will be emitted to syslog
> + warning about the lack of a pipe handler.
>
> ==============================================================
>
> diff --git a/fs/exec.c b/fs/exec.c
> index da27b91..715d7cf 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -2106,10 +2106,8 @@ void do_coredump(long signr, int exit_code, struct pt_regs *regs)
> struct core_name cn;
> struct mm_struct *mm = current->mm;
> struct linux_binfmt * binfmt;
> - const struct cred *old_cred;
> - struct cred *cred;
> + bool pipeonly = false;
> int retval = 0;
> - int flag = 0;
> int ispipe;
> static atomic_t core_dump_count = ATOMIC_INIT(0);
> struct coredump_params cprm = {
> @@ -2132,25 +2130,16 @@ void do_coredump(long signr, int exit_code, struct pt_regs *regs)
> if (!__get_dumpable(cprm.mm_flags))
> goto fail;
>
> - cred = prepare_creds();
> - if (!cred)
> - goto fail;
> /*
> - * We cannot trust fsuid as being the "true" uid of the
> - * process nor do we know its entire history. We only know it
> - * was tainted so we dump it as root in mode 2.
> + * We cannot trust the environment when dumping in mode 2, so only
> + * write the dump to a pipe.
> */
> - if (__get_dumpable(cprm.mm_flags) == 2) {
> - /* Setuid core dump mode */
> - flag = O_EXCL; /* Stop rewrite attacks */
> - cred->fsuid = GLOBAL_ROOT_UID; /* Dump root private */
> - }
> + if (__get_dumpable(cprm.mm_flags) == 2)
> + pipeonly = true;
>
> retval = coredump_wait(exit_code, &core_state);
> if (retval < 0)
> - goto fail_creds;
> -
> - old_cred = override_creds(cred);
> + goto fail;
>
> /*
> * Clear any false indication of pending signals that might
> @@ -2220,11 +2209,20 @@ void do_coredump(long signr, int exit_code, struct pt_regs *regs)
> } else {
> struct inode *inode;
>
> + if (pipeonly) {
> + printk(KERN_WARNING "Pid %d(%s) could not dump core "\
> + "without a core dump pipe handler defined "\
> + "via the core_pattern sysctl!\n",
> + task_tgid_vnr(current), current->comm);
> + printk(KERN_WARNING "Skipping core dump\n");
> + goto fail_unlock;
> + }
> +
> if (cprm.limit < binfmt->min_coredump)
> goto fail_unlock;
>
> cprm.file = filp_open(cn.corename,
> - O_CREAT | 2 | O_NOFOLLOW | O_LARGEFILE | flag,
> + O_CREAT | 2 | O_NOFOLLOW | O_LARGEFILE,
> 0600);
> if (IS_ERR(cprm.file))
> goto fail_unlock;
> @@ -2268,9 +2266,6 @@ fail_unlock:
> kfree(cn.corename);
> fail_corename:
> coredump_finish(mm);
> - revert_creds(old_cred);
> -fail_creds:
> - put_cred(cred);
> fail:
> return;
> }


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