[PATCH 5/8] hw_breakpoint: Add perf_event_attr fields check in __modify_user_hw_breakpoint

[Jiri Olsa]

And rename it to modify_user_hw_breakpoint_check.

We are about to use modify_user_hw_breakpoint_check for user space
breakpoints modification, we must be very strict to check only the
fields we can change have changed. As Peter explained:

  Suppose someone does:
        attr = malloc(sizeof(*attr)); // uninitialized memory
        attr->type = BP;
        attr->bp_addr = new_addr;
        attr->bp_type = bp_type;
        attr->bp_len = bp_len;
        ioctl(fd, PERF_IOC_MOD_ATTR, &attr);

  And feeds absolute shite for the rest of the fields.
  Then we later want to extend IOC_MOD_ATTR to allow changing
  attr::sample_type but we can't, because that would break the
  above application.

I'm making this check optional because we already export
modify_user_hw_breakpoint and with this check we could
break existing users.

Suggested-by: Peter Zijlstra <pet...@infradead.org>
Signed-off-by: Jiri Olsa <jo...@kernel.org>
---
 kernel/events/hw_breakpoint.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/kernel/events/hw_breakpoint.c b/kernel/events/hw_breakpoint.c
index a556aba223da..0c82663395f7 100644
--- a/kernel/events/hw_breakpoint.c
+++ b/kernel/events/hw_breakpoint.c
@@ -456,7 +456,9 @@ register_user_hw_breakpoint(struct perf_event_attr *attr,
 }
 EXPORT_SYMBOL_GPL(register_user_hw_breakpoint);
 
-static int __modify_user_hw_breakpoint(struct perf_event *bp, struct 
perf_event_attr *attr)
+static int
+modify_user_hw_breakpoint_check(struct perf_event *bp, struct perf_event_attr 
*attr,
+                               bool check)
 {
        u64 old_addr = bp->attr.bp_addr;
        u64 old_len  = bp->attr.bp_len;
@@ -468,6 +470,9 @@ static int __modify_user_hw_breakpoint(struct perf_event 
*bp, struct perf_event_
        bp->attr.bp_type = attr->bp_type;
        bp->attr.bp_len  = attr->bp_len;
 
+       if (check && memcmp(&bp->attr, attr, sizeof(*attr)))
+               return -EINVAL;
+
        err = validate_hw_breakpoint(bp);
        if (!err && modify)
                err = modify_bp_slot(bp, old_type);
@@ -505,7 +510,7 @@ int modify_user_hw_breakpoint(struct perf_event *bp, struct 
perf_event_attr *att
        else
                perf_event_disable(bp);
 
-       err = __modify_user_hw_breakpoint(bp, attr);
+       err = modify_user_hw_breakpoint_check(bp, attr, false);
 
        if (err) {
                if (!bp->attr.disabled)
-- 
2.13.6