[PATCH v2] mm,procfs: allow read-only remote mm access under CAP_PERFMON

[Andrii Nakryiko <andrii@kernel.org>]

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.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
---
v1->v2:
  - expanded commit message a bit more about PTRACE_MODE_ATTACH vs
    PTRACE_MODE_READ uses inside procfs; left the generic logic untouched, as
    it still seems generally meaningful to allow CAP_PERFMON for read-only
    memory access, given its use within perf and BPF subsystems;
  - moved perfmon_capable() check after ptrace_may_access() to minimize the
    worry of extra audit messages where CAP_SYS_PTRACE would be provided
    (Christian);
  - s/can/may/_access_mm rename (Kees);

 kernel/fork.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/kernel/fork.c b/kernel/fork.c
index ded49f18cd95..452018f752a1 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1547,6 +1547,17 @@ 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;
@@ -1559,7 +1570,7 @@ 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);
 	}
-- 
2.43.5