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Subject[PATCH 5/8] hw_breakpoint: Add perf_event_attr fields check in __modify_user_hw_breakpoint
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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 <peterz@infradead.org>
Signed-off-by: Jiri Olsa <jolsa@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
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