On 27/02/2024 17:26, Kees Cook wrote:
On Tue, Feb 27, 2024 at 04:37:36PM +0100, Daniel Lezcano wrote:
On 27/02/2024 12:09, Rafael J. Wysocki wrote:
On Tue, Feb 27, 2024 at 11:14 AM Daniel Lezcano
<[email protected]> wrote:
On 27/02/2024 01:54, Nathan Chancellor wrote:
When booting a CONFIG_FORTIFY_SOURCE=y kernel compiled with a toolchain
that supports __counted_by() (such as clang-18 and newer), there is a
panic on boot:
[ 2.913770] memcpy: detected buffer overflow: 72 byte write of buffer
size 0
[ 2.920834] WARNING: CPU: 2 PID: 1 at lib/string_helpers.c:1027
__fortify_report+0x5c/0x74
...
[ 3.039208] Call trace:
[ 3.041643] __fortify_report+0x5c/0x74
[ 3.045469] __fortify_panic+0x18/0x20
[ 3.049209] thermal_zone_device_register_with_trips+0x4c8/0x4f8
This panic occurs because trips is counted by num_trips but num_trips is
assigned after the call to memcpy(), so the fortify checks think the
buffer size is zero because tz was allocated with kzalloc().
Move the num_trips assignment before the memcpy() to resolve the panic
and ensure that the fortify checks work properly.
Fixes: 9b0a62758665 ("thermal: core: Store zone trips table in struct
thermal_zone_device")
Signed-off-by: Nathan Chancellor <[email protected]>
---
drivers/thermal/thermal_core.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/drivers/thermal/thermal_core.c b/drivers/thermal/thermal_core.c
index bb21f78b4bfa..1eabc8ebe27d 100644
--- a/drivers/thermal/thermal_core.c
+++ b/drivers/thermal/thermal_core.c
@@ -1354,8 +1354,8 @@ thermal_zone_device_register_with_trips(const char *type,
tz->device.class = thermal_class;
tz->devdata = devdata;
- memcpy(tz->trips, trips, num_trips * sizeof(*trips));
tz->num_trips = num_trips;
+ memcpy(tz->trips, trips, num_trips * sizeof(*trips));
IIUC, clang-18 is used and supports __counted_by().
Is it possible sizeof(*trips) returns already the real trips array size
and we are multiplying it again by num_trips ?
While with an older compiler, __counted_by() does nothing and we have to
multiply by num_trips ?
IOW, the array size arithmetic is different depending if we have
_counted_by supported or not ?
IIUC it is just the instrumentation using the current value of
tz->num_trips (which is 0 before the initialization).
Right, but I am wondering if
memcpy(tz->trips, trips, num_trips * sizeof(*trips));
is still correct with __counted_by because:
(1) if the compiler supports it:
sizeof(*trips) == 24 bytes * num_trips
I think you're misunderstanding. The above sizeof() only evaluates a
single instance -- it has no idea how many more there may be.
Specifically:
sizeof(*trips) == sizeof(struct thermal_trip)
then:
memcpy(tz->trips, trips, num_trips * sizeof(*trips));
memcpy(tz->trips, trips, num_trips * 24 * num_trips);
==> memory size = 24 * num_trips^2
It's not counted twice. Under CONFIG_FORTIFY_SOURCE=y, memcpy is a macro
that expands to a checking routine (see include/linux/fortify-string.h),
which is using __builtin_dynamic_object_size() to determine the
available size of the destination buffer (tz->trips). Specifically:
__builtin_dynamic_object_size(tz->trips)
When __bdos evaluates a flexible array (i.e. tz->trips), it will see the
associated 'counted_by' attribute, and go look up the value of the
designated struct member (tz->num_trips). It then calculates:
sizeof(*tz->trips) /* a single instance */
*
tz->num_trips
Ok my misunderstanding was I thought sizeof() was calling _bdos under
the hood, so when calling sizeof(flex_array), it was returning the
computed size inferring from the __counted_by field.
Before the patch, tz->num_trips is 0, so the destination buffer size
appears to be of size 0 bytes. After the patch, it contains the
same value as the "num_trips" function argument, so the destination
buffer appears to be the matching size of "num_trips * sizeof(struct
thermal_trip)".
Hopefully that helps! If not, I can try again. :)
It is ok thanks for the clarification
--
<http://www.linaro.org/> Linaro.org │ Open source software for ARM SoCs
Follow Linaro: <http://www.facebook.com/pages/Linaro> Facebook |
<http://twitter.com/#!/linaroorg> Twitter |
<http://www.linaro.org/linaro-blog/> Blog
