On 3/4/26 5:52 PM, Sayali Patil wrote:
On powerpc with PREEMPT_FULL or PREEMPT_LAZY and function tracing enabled,
KUAP warnings can be triggered from the VMX usercopy path under memory
stress workloads.
KUAP requires that no subfunctions are called once userspace access has
been enabled. The existing VMX copy implementation violates this
requirement by invoking enter_vmx_usercopy() from the assembly path after
userspace access has already been enabled. If preemption occurs
in this window, the AMR state may not be preserved correctly,
leading to unexpected userspace access state and resulting in
KUAP warnings.
Fix this by restructuring the VMX usercopy flow so that VMX selection
and VMX state management are centralized in raw_copy_tofrom_user(),
which is invoked by the raw_copy_{to,from,in}_user() wrappers.
The new flow is:
- raw_copy_{to,from,in}_user() calls raw_copy_tofrom_user()
- raw_copy_tofrom_user() decides whether to use the VMX path
based on size and CPU capability
- Call enter_vmx_usercopy() before enabling userspace access
- Enable userspace access as per the copy direction
and perform the VMX copy
- Disable userspace access as per the copy direction
- Call exit_vmx_usercopy()
- Fall back to the base copy routine if the VMX copy faults
With this change, the VMX assembly routines no longer perform VMX state
management or call helper functions; they only implement the
copy operations.
The previous feature-section based VMX selection inside
__copy_tofrom_user_power7() is removed, and a dedicated
__copy_tofrom_user_power7_vmx() entry point is introduced.
This ensures correct KUAP ordering, avoids subfunction calls
while KUAP is unlocked, and eliminates the warnings while preserving
the VMX fast path.
Fixes: de78a9c42a79 ("powerpc: Add a framework for Kernel Userspace Access
Protection")
Reported-by: Shrikanth Hegde <[email protected]>
Closes:
https://lore.kernel.org/all/[email protected]/
Suggested-by: Christophe Leroy (CS GROUP) <[email protected]>
Reviewed-by: Christophe Leroy (CS GROUP) <[email protected]>
Co-developed-by: Aboorva Devarajan <[email protected]>
Signed-off-by: Aboorva Devarajan <[email protected]>
Signed-off-by: Sayali Patil <[email protected]>
---
v3->v4
- Addressd as per review feedback for some editorial comments.
v3: https://lore.kernel.org/all/[email protected]/
---
arch/powerpc/include/asm/uaccess.h | 65 +++++++++++++++++++++---------
arch/powerpc/lib/copyuser_64.S | 1 +
arch/powerpc/lib/copyuser_power7.S | 45 +++++++--------------
arch/powerpc/lib/vmx-helper.c | 2 +
4 files changed, 63 insertions(+), 50 deletions(-)
This does fix the reported problem. Gave this a try on PowerNV box too.
Tested-by: Shrikanth Hegde <[email protected]>
