From: Andrii Nakryiko Date: Mon, 27 Jan 2025 22:21:14 +0000 (-0800) Subject: mm,procfs: allow read-only remote mm access under CAP_PERFMON X-Git-Tag: v6.15-rc1~80^2~76 X-Git-Url: https://www.infradead.org/git/?a=commitdiff_plain;h=0c555a3c1bc9114ad91422b941dcd29e02490687;p=linux.git mm,procfs: allow read-only remote mm access under CAP_PERFMON It's very common for various tracing and profiling toolis to need to access /proc/PID/maps contents for stack symbolization needs to learn which shared libraries are mapped in memory, at which file offset, etc. Currently, access to /proc/PID/maps requires CAP_SYS_PTRACE (unless we are looking at data for our own process, which is a trivial case not too relevant for profilers use cases). Unfortunately, CAP_SYS_PTRACE implies way more than just ability to discover memory layout of another process: it allows to fully control arbitrary other processes. This is problematic from security POV for applications that only need read-only /proc/PID/maps (and other similar read-only data) access, and in large production settings CAP_SYS_PTRACE is frowned upon even for the system-wide profilers. On the other hand, it's already possible to access similar kind of information (and more) with just CAP_PERFMON capability. E.g., setting up PERF_RECORD_MMAP collection through perf_event_open() would give one similar information to what /proc/PID/maps provides. CAP_PERFMON, together with CAP_BPF, is already a very common combination for system-wide profiling and observability application. As such, it's reasonable and convenient to be able to access /proc/PID/maps with CAP_PERFMON capabilities instead of CAP_SYS_PTRACE. For procfs, these permissions are checked through common mm_access() helper, and so we augment that with cap_perfmon() check *only* if requested mode is PTRACE_MODE_READ. I.e., PTRACE_MODE_ATTACH wouldn't be permitted by CAP_PERFMON. So /proc/PID/mem, which uses PTRACE_MODE_ATTACH, won't be permitted by CAP_PERFMON, but /proc/PID/maps, /proc/PID/environ, and a bunch of other read-only contents will be allowable under CAP_PERFMON. Besides procfs itself, mm_access() is used by process_madvise() and process_vm_{readv,writev}() syscalls. The former one uses PTRACE_MODE_READ to avoid leaking ASLR metadata, and as such CAP_PERFMON seems like a meaningful allowable capability as well. process_vm_{readv,writev} currently assume PTRACE_MODE_ATTACH level of permissions (though for readv PTRACE_MODE_READ seems more reasonable, but that's outside the scope of this change), and as such won't be affected by this patch. Link: https://lkml.kernel.org/r/20250127222114.1132392-1-andrii@kernel.org Signed-off-by: Andrii Nakryiko Reviewed-by: Shakeel Butt Acked-by: Ingo Molnar Cc: Al Viro Cc: Christian Brauner Cc: Jann Horn Cc: Kees Cook Cc: Liam Howlett Cc: "Mike Rapoport (IBM)" Cc: Peter Zijlstra (Intel) Cc: Suren Baghdasaryan Signed-off-by: Andrew Morton --- diff --git a/kernel/fork.c b/kernel/fork.c index 735405a9c5f3..7757e74ebce4 100644 --- a/kernel/fork.c +++ b/kernel/fork.c @@ -1559,6 +1559,17 @@ struct mm_struct *get_task_mm(struct task_struct *task) } EXPORT_SYMBOL_GPL(get_task_mm); +static bool may_access_mm(struct mm_struct *mm, struct task_struct *task, unsigned int mode) +{ + if (mm == current->mm) + return true; + if (ptrace_may_access(task, mode)) + return true; + if ((mode & PTRACE_MODE_READ) && perfmon_capable()) + return true; + return false; +} + struct mm_struct *mm_access(struct task_struct *task, unsigned int mode) { struct mm_struct *mm; @@ -1571,7 +1582,7 @@ struct mm_struct *mm_access(struct task_struct *task, unsigned int mode) mm = get_task_mm(task); if (!mm) { mm = ERR_PTR(-ESRCH); - } else if (mm != current->mm && !ptrace_may_access(task, mode)) { + } else if (!may_access_mm(mm, task, mode)) { mmput(mm); mm = ERR_PTR(-EACCES); }