Stephen, On 06/23/2016 08:05 PM, Stephen Smalley wrote:
On 06/21/2016 05:41 AM, Michael Kerrisk (man-pages) wrote:Hi Jann, Stephen, et al. Jann, since you recently committed a patch in this area, and Stephen, since you committed 006ebb40d3d much further back in time, I wonder if you might help me by reviewing the text below that I propose to add to the ptrace(2) man page, in order to document "ptrace access mode checking" that is performed in various parts of the kernel-user-space interface. Of course, I welcome input from anyone else as well. Here's the new ptrace(2) text. Any comments, technical or terminological fixes, other improvements, etc. are welcome. [[ Ptrace access mode checking Various parts of the kernel-user-space API (not just ptrace(2) operations), require so-called "ptrace access mode permissions" which are gated by Linux Security Modules (LSMs) such as SELinux, Yama, Smack, or the default LSM. Prior to Linux 2.6.27, all such checks were of a single type. Since Linux 2.6.27, two access mode levels are distinguished: PTRACE_MODE_READ For "read" operations or other operations that are less dangerous, such as: get_robust_list(2); kcmp(2); reading /proc/[pid]/auxv, /proc/[pid]/environ, or /proc/[pid]/stat; or readlink(2) of a /proc/[pid]/ns/* file. PTRACE_MODE_ATTACH For "write" operations, or other operations that are more dangerous, such as: ptrace attaching (PTRACE_ATTACH) to another process or calling process_vm_writev(2). (PTRACE_MODE_ATTACH was effec‐ tively the default before Linux 2.6.27.)That was the intent when the distinction was introduced, but it doesn't appear to have been properly maintained, e.g. there is now a common helper lock_trace() that is used for /proc/pid/{stack,syscall,personality} but checks PTRACE_MODE_ATTACH, and PTRACE_MODE_ATTACH is also used in timerslack_ns_write/show(). Likely should review and make them consistent. There was also some debate about proper handling of /proc/pid/fd. Arguably that one might belong back in the _ATTACH camp.
Thanks for the background info.
Since Linux 4.5, the above access mode checks may be combined (ORed) with one of the following modifiers: PTRACE_MODE_FSCREDS Use the caller's filesystem UID and GID (see creden‐ tials(7)) or effective capabilities for LSM checks. PTRACE_MODE_REALCREDS Use the caller's real UID and GID or permitted capabili‐ ties for LSM checks. This was effectively the default before Linux 4.5. Because combining one of the credential modifiers with one of the aforementioned access modes is typical, some macros are defined in the kernel sources for the combinations: PTRACE_MODE_READ_FSCREDS Defined as PTRACE_MODE_READ | PTRACE_MODE_FSCREDS. PTRACE_MODE_READ_REALCREDS Defined as PTRACE_MODE_READ | PTRACE_MODE_REALCREDS. PTRACE_MODE_ATTACH_FSCREDS Defined as PTRACE_MODE_ATTACH | PTRACE_MODE_FSCREDS. PTRACE_MODE_ATTACH_REALCREDS Defined as PTRACE_MODE_ATTACH | PTRACE_MODE_REALCREDS. One further modifier can be ORed with the access mode: PTRACE_MODE_NOAUDIT (since Linux 3.3) Don't audit this access mode check. [I'd quite welcome some text to explain "auditing" here.]Some ptrace access mode checks, such as checks when reading /proc/pid/stat, merely cause the output to be filtered/sanitized rather than an error to be returned to the caller. In these cases, accessing the file is not a security violation and there is no reason to generate a security audit record. This modifier suppresses the generation of such an audit record for the particular access check.
Thanks, I've added that text to the man page more or less as you gave it here. Cheers, Michael -- Michael Kerrisk Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ Linux/UNIX System Programming Training: http://man7.org/training/ -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
