[PATCH v4 23/23] bpf: Fail bpf_probe_write_user() while mm is switched

[Rick Edgecombe]

From: Nadav Amit <na...@vmware.com>

When using a temporary mm, bpf_probe_write_user() should not be able to
write to user memory, since user memory addresses may be used to map
kernel memory.  Detect these cases and fail bpf_probe_write_user() in
such cases.

Cc: Daniel Borkmann <dan...@iogearbox.net>
Cc: Alexei Starovoitov <a...@kernel.org>
Reported-by: Jann Horn <ja...@google.com>
Suggested-by: Jann Horn <ja...@google.com>
Signed-off-by: Nadav Amit <na...@vmware.com>
Signed-off-by: Rick Edgecombe <rick.p.edgeco...@intel.com>
---
 kernel/trace/bpf_trace.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
index d64c00afceb5..94b0e37d90ef 100644
--- a/kernel/trace/bpf_trace.c
+++ b/kernel/trace/bpf_trace.c
@@ -14,6 +14,8 @@
 #include <linux/syscalls.h>
 #include <linux/error-injection.h>
 
+#include <asm/tlb.h>
+
 #include "trace_probe.h"
 #include "trace.h"
 
@@ -163,6 +165,10 @@ BPF_CALL_3(bpf_probe_write_user, void *, unsafe_ptr, const 
void *, src,
         * access_ok() should prevent writing to non-user memory, but in
         * some situations (nommu, temporary switch, etc) access_ok() does
         * not provide enough validation, hence the check on KERNEL_DS.
+        *
+        * nmi_uaccess_okay() ensures the probe is not run in an interim
+        * state, when the task or mm are switched. This is specifically
+        * required to prevent the use of temporary mm.
         */
 
        if (unlikely(in_interrupt() ||
@@ -170,6 +176,8 @@ BPF_CALL_3(bpf_probe_write_user, void *, unsafe_ptr, const 
void *, src,
                return -EPERM;
        if (unlikely(uaccess_kernel()))
                return -EPERM;
+       if (unlikely(!nmi_uaccess_okay()))
+               return -EPERM;
        if (!access_ok(unsafe_ptr, size))
                return -EPERM;
 
-- 
2.17.1