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@xxxxxxxxxx>
---
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
