On Tue, Jun 21, 2016 at 11:41:16AM +0200, 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. As others have said, I'm surprised about seeing documentation about kernel-internal constants in manpages - but I think it might be a good thing to have there, given that people who look at ptrace(2) are likely to be interested in low-level details. > [[ > 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.) > > Since Linux 4.5, the above access mode checks may be combined s/may/must/; otherwise __ptrace_may_access() will yell about the kernel code being broken and deny access. > (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.] > > The algorithm employed for ptrace access mode checking deter‐ > mines whether the calling process is allowed to perform the > corresponding action on the target process, as follows: > > 1. If the calling thread and the target thread are in the same > thread group, access is always allowed. > > 2. If the access mode specifies PTRACE_MODE_FSCREDS, then for > the check in the next step, employ the caller's filesystem > user ID and group ID (see credentials(7)); otherwise (the > access mode specifies PTRACE_MODE_REALCREDS, so) use the > caller's real user ID and group ID. Might want to add a "for historical reasons" or so here. > 3. Deny access if neither of the following is true: > > · The real, effective, and saved-set user IDs of the target > match the caller's user ID, and the real, effective, and > saved-set group IDs of the target match the caller's > group ID. > > · The caller has the CAP_SYS_PTRACE capability. Might want to also specify here (like below) that the caller needs to have the capability relative to the user ns of the target. > 4. Deny access if the target process "dumpable" attribute has > a value other than 1 (SUID_DUMP_USER; see the discussion of > PR_SET_DUMPABLE in prctl(2)), and the caller does not have > the CAP_SYS_PTRACE capability in the user namespace of the > target process. > > 5. The kernel LSM security_ptrace_access_check() interface is > invoked to see if ptrace access is permitted. The results > depend on the LSM. The implementation of this interface in > the default LSM performs the following steps: For people who are unaware of how the LSM API works, it might be good to clarify that the commoncap LSM is *always* invoked; otherwise, it might give the impression that using another LSM would replace it. (Also, are there other documents that refer to it as "default LSM"? I think that that term is slightly confusing.) > a) If the access mode includes PTRACE_MODE_FSCREDS, then > use the caller's effective capability set in the follow‐ > ing check; otherwise (the access mode specifies > PTRACE_MODE_REALCREDS, so) use the caller's permitted > capability set. > > b) Deny access if neither of the following is true: > > · The caller's capabilities are a proper superset of the > target process's permitted capabilities. This also requires the caller and the target to be in the same user namespace. > · The caller has the CAP_SYS_PTRACE capability in the > target process's user namespace. > > Note that the default LSM does not distinguish between > PTRACE_MODE_READ and PTRACE_MODE_ATTACH. > > 6. If access has not been denied by any of the preceding > steps, then access is allowed. > ]] > > There are accompanying changes to various pages that refer to > the new text in ptrace(2), so that, for example, kcmp(2) adds: > > Permission to employ kcmp() is governed by ptrace access mode > PTRACE_MODE_ATTACH_REALCREDS checks against both pid1 and pid2; > see ptrace(2). (Actually, kcmp() just needs READ access - you described that accurately earlier, but it's wrong here.) > and proc.5 has additions such as: > > /proc/[pid]/auxv (since 2.6.0-test7) > ... > Permission to access this file is governed by a ptrace > access mode PTRACE_MODE_READ_FSCREDS check; see > ptrace(2). > > /proc/[pid]/cwd > ... > Permission to dereference or read (readlink(2)) this > symbolic link is governed by a ptrace access mode > PTRACE_MODE_READ_FSCREDS check; see ptrace(2). That sounds great! :)
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