[PATCH 0/2] Input: synaptics-rmi4 - fix two device-controlled out-of-bounds writes

[Wei Jie Law]

Hi Dmitry,

While putting together a reproducer for an out-of-bounds bug in hid-rmi
(v2 posted separately, [1]) I found two further memory-safety problems in
the shared RMI4 core.  Both are driven entirely by data the *device*
supplies -- its Page Description Table -- so they are reachable from a
malicious USB HID device with no code running on the victim, and equally
from I2C and SMBus RMI4 devices.  Neither depends on the hid-rmi bug;
they are in drivers/input/rmi4/ and need fixing separately.

Both are present in mainline and in every stable tree I looked at.

1/2 is an off-by-one: RMI_PDT_INT_SOURCE_COUNT_MASK is 0x07, so
interrupt_source_count can be 7, but struct rmi_function declares
int irq[RMI_FN_MAX_IRQS] with RMI_FN_MAX_IRQS == 6 and two loops walk it
up to fn->num_of_irqs.  irq[6] is the storage of the next member,
unsigned int irq_pos, so the function's position in the interrupt bitmap
is silently replaced with a Linux virq number.  UBSAN flags all five
stores plus the read on the unregister path.

2/2 is a time-of-check/time-of-use across two reads of the device: the
PDT is scanned three times and re-read from the device every time,
irq_mask[] is sized by the counting scan and filled by the creating scan,
and nothing verifies the two agree.  KASAN catches the resulting
set_bit() walking off the end of the flexible array at the tail of every
struct rmi_function.

How this was verified
---------------------

Linux v6.12.69 (CONFIG_UBSAN_BOUNDS=y, booted slub_debug=FZPU) and
v6.12.105 (CONFIG_KASAN=y + CONFIG_KASAN_INLINE=y, CONFIG_UBSAN_BOUNDS=y,
booted kasan_multi_shot -- generic KASAN otherwise reports only the first
error per boot), x86_64.  An emulated Synaptics RMI4 device publishes a
Page Description Table crafted for each case.  Two independent
reproducers, giving identical results:

  - a /dev/uhid program -- no hardware, fully deterministic, and the easy
    one to run;
  - the same device over dummy_hcd + raw-gadget with Facedancer, so the
    reports really traverse usbcore -> usbhid -> hid-rmi.

Each bug was exercised on its own cold boot, because heap state left by a
previous run changes what the out-of-bounds read returns and UBSAN
reports each call site only once per boot.

With both patches applied 1/2 produces no UBSAN reports and the same
device -- F01 declaring the full 7 interrupt sources -- probes normally,
and 2/2 fails the probe cleanly instead of corrupting the heap.

I am happy to post the reproducers, or to send them privately if you
would rather they did not go to a public list.

[1] https://patchwork.kernel.org/project/linux-input/patch/00a489f38b240624dcb5a4bae36a53fcba9cfb47.1787549195.git.98lawweijie@xxxxxxxxx/

Wei Jie Law (2):
  Input: synaptics-rmi4 - fix irq[] overrun with 7 interrupt sources
  Input: synaptics-rmi4 - reject a PDT that grows between scans

 drivers/input/rmi4/rmi_bus.h    |  9 ++++++---
 drivers/input/rmi4/rmi_driver.c | 18 ++++++++++++++++++
 2 files changed, 24 insertions(+), 3 deletions(-)

-- 
2.43.0