In the Linux kernel, the following vulnerability has been resolved:
powerpc: Do not restore KUAP in arch_exit_to_user_mode_prepare()
KUAP means kernel cannot touch user memory unless it explicitly is
enabled. In the kernel it should stay AMR_KUAP_BLOCKED. While returning
to userspace just before RFI, kernel should restore the user AMR value
back.
Looks like GENERIC_ENTRY might be treating arch_exit_to_user_mode_prepare()
as the last architecture step before returning to userspace.
commit bee25f97ad24 ("powerpc: Enable GENERIC_ENTRY feature")
therefore called kuap_user_restore() from that hook. But on PowerPC that
is too early. After irqentry_exit() / syscall_exit_to_user_mode() we
still run platform specific exit routines.
e.g. code snippets showing both exception handling and system call
handling as the callers of function arch_exit_to_user_mode_prepare()
which does kuap_user_restore(). The below path shows that calling
kuap_user_restore() is too early when called from
arch_exit_to_user_mode_prepare().
Exception handling in exceptions-64s.S
=======================================
bl CFUNC(do_page_fault)
..DEFINE_INTERRUPT_HANDLER_ASYNC(do_page_fault)
arch_interrupt_async_enter_prepare(regs);
state = irqentry_enter(regs);
instrumentation_begin();
irq_enter_rcu();
handler(regs);
nap_adjust_return(regs);
irq_exit_rcu();
instrumentation_end();
arch_interrupt_async_exit_prepare(regs);
irqentry_exit(regs, state); <<< too early
irqentry_exit_to_user_mode()
__exit_to_user_mode_prepare(regs, EXIT_TO_USER_MODE_WORK_IRQ);
arch_exit_to_user_mode_prepare(regs, ti_work); <<< too early
b interrupt_return_srr
.. bl CFUNC(interrupt_exit_user_prepare) <<< already calls kuap_user_restore
prep_irq_for_enabled_exit() retry can run kernel code with IRQs on. So
only when that routine is fully finished is when the user KUAP should be
fully restored which interrupt_exit_user_prepare() already takes care of
before returning.
Similarly for system call handling in interrupt_64.S
======================================================
bl CFUNC(system_call_exception)
.Lsyscall_exit:
addi r4,r1,STACK_INT_FRAME_REGS
li r5,0 /* !scv */
bl CFUNC(syscall_exit_prepare)
.. kuap_assert_locked();
syscall_exit_to_user_mode(regs); <<< too early
syscall_exit_to_user_mode_prepare(regs); <<< too early
kuap_user_restore(regs); <<< already calls
syscall_exit_prepare(), which can enable IRQs, replay a pending
interrupt, and only then rfi. Those functions already restore KUAP
immediately before rfi.
Note that if we restore the user AMR too early like in the current code
as shown from the code snippets above, then we get the following warning
when CONFIG_PPC_KUAP_DEBUG is enabled:
WARNING: arch/powerpc/include/asm/book3s/64/kup.h:293 at interrupt_exit_user_prepare+0x1a0/0x1c0
Hardware name: IBM pSeries (emulated by qemu) POWER10 (architected)
TRAP: 0700
LR: c00000000000d8d4 CTR: c0000000021fe500
MSR: <SF,EE,ME,IR,DR,RI,LE> CR: 44000804 XER: 20040000
interrupt_exit_user_prepare+0x1a0/0x1c0
interrupt_return_srr_user+0x8/0x12c
powerpc: Do not restore KUAP in arch_exit_to_user_mode_prepare()
KUAP means kernel cannot touch user memory unless it explicitly is
enabled. In the kernel it should stay AMR_KUAP_BLOCKED. While returning
to userspace just before RFI, kernel should restore the user AMR value
back.
Looks like GENERIC_ENTRY might be treating arch_exit_to_user_mode_prepare()
as the last architecture step before returning to userspace.
commit bee25f97ad24 ("powerpc: Enable GENERIC_ENTRY feature")
therefore called kuap_user_restore() from that hook. But on PowerPC that
is too early. After irqentry_exit() / syscall_exit_to_user_mode() we
still run platform specific exit routines.
e.g. code snippets showing both exception handling and system call
handling as the callers of function arch_exit_to_user_mode_prepare()
which does kuap_user_restore(). The below path shows that calling
kuap_user_restore() is too early when called from
arch_exit_to_user_mode_prepare().
Exception handling in exceptions-64s.S
=======================================
bl CFUNC(do_page_fault)
..DEFINE_INTERRUPT_HANDLER_ASYNC(do_page_fault)
arch_interrupt_async_enter_prepare(regs);
state = irqentry_enter(regs);
instrumentation_begin();
irq_enter_rcu();
handler(regs);
nap_adjust_return(regs);
irq_exit_rcu();
instrumentation_end();
arch_interrupt_async_exit_prepare(regs);
irqentry_exit(regs, state); <<< too early
irqentry_exit_to_user_mode()
__exit_to_user_mode_prepare(regs, EXIT_TO_USER_MODE_WORK_IRQ);
arch_exit_to_user_mode_prepare(regs, ti_work); <<< too early
b interrupt_return_srr
.. bl CFUNC(interrupt_exit_user_prepare) <<< already calls kuap_user_restore
prep_irq_for_enabled_exit() retry can run kernel code with IRQs on. So
only when that routine is fully finished is when the user KUAP should be
fully restored which interrupt_exit_user_prepare() already takes care of
before returning.
Similarly for system call handling in interrupt_64.S
======================================================
bl CFUNC(system_call_exception)
.Lsyscall_exit:
addi r4,r1,STACK_INT_FRAME_REGS
li r5,0 /* !scv */
bl CFUNC(syscall_exit_prepare)
.. kuap_assert_locked();
syscall_exit_to_user_mode(regs); <<< too early
syscall_exit_to_user_mode_prepare(regs); <<< too early
kuap_user_restore(regs); <<< already calls
syscall_exit_prepare(), which can enable IRQs, replay a pending
interrupt, and only then rfi. Those functions already restore KUAP
immediately before rfi.
Note that if we restore the user AMR too early like in the current code
as shown from the code snippets above, then we get the following warning
when CONFIG_PPC_KUAP_DEBUG is enabled:
WARNING: arch/powerpc/include/asm/book3s/64/kup.h:293 at interrupt_exit_user_prepare+0x1a0/0x1c0
Hardware name: IBM pSeries (emulated by qemu) POWER10 (architected)
TRAP: 0700
LR: c00000000000d8d4 CTR: c0000000021fe500
MSR: <SF,EE,ME,IR,DR,RI,LE> CR: 44000804 XER: 20040000
interrupt_exit_user_prepare+0x1a0/0x1c0
interrupt_return_srr_user+0x8/0x12c
Default status is the baseline for the product, each version can override it (e.g. patched versions marked unaffected).
| Vendor | Product | Default status | Versions | |||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Linux | Linux | unaffected |
|
|||||||||||||||
| Linux | Linux | affected |
|
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Fixes
Solution
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Workaround
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References
History
Wed, 14 Oct 2026 13:45:00 +0000
| Type | Values Removed | Values Added |
|---|---|---|
| Description | In the Linux kernel, the following vulnerability has been resolved: powerpc: Do not restore KUAP in arch_exit_to_user_mode_prepare() KUAP means kernel cannot touch user memory unless it explicitly is enabled. In the kernel it should stay AMR_KUAP_BLOCKED. While returning to userspace just before RFI, kernel should restore the user AMR value back. Looks like GENERIC_ENTRY might be treating arch_exit_to_user_mode_prepare() as the last architecture step before returning to userspace. commit bee25f97ad24 ("powerpc: Enable GENERIC_ENTRY feature") therefore called kuap_user_restore() from that hook. But on PowerPC that is too early. After irqentry_exit() / syscall_exit_to_user_mode() we still run platform specific exit routines. e.g. code snippets showing both exception handling and system call handling as the callers of function arch_exit_to_user_mode_prepare() which does kuap_user_restore(). The below path shows that calling kuap_user_restore() is too early when called from arch_exit_to_user_mode_prepare(). Exception handling in exceptions-64s.S ======================================= bl CFUNC(do_page_fault) ..DEFINE_INTERRUPT_HANDLER_ASYNC(do_page_fault) arch_interrupt_async_enter_prepare(regs); state = irqentry_enter(regs); instrumentation_begin(); irq_enter_rcu(); handler(regs); nap_adjust_return(regs); irq_exit_rcu(); instrumentation_end(); arch_interrupt_async_exit_prepare(regs); irqentry_exit(regs, state); <<< too early irqentry_exit_to_user_mode() __exit_to_user_mode_prepare(regs, EXIT_TO_USER_MODE_WORK_IRQ); arch_exit_to_user_mode_prepare(regs, ti_work); <<< too early b interrupt_return_srr .. bl CFUNC(interrupt_exit_user_prepare) <<< already calls kuap_user_restore prep_irq_for_enabled_exit() retry can run kernel code with IRQs on. So only when that routine is fully finished is when the user KUAP should be fully restored which interrupt_exit_user_prepare() already takes care of before returning. Similarly for system call handling in interrupt_64.S ====================================================== bl CFUNC(system_call_exception) .Lsyscall_exit: addi r4,r1,STACK_INT_FRAME_REGS li r5,0 /* !scv */ bl CFUNC(syscall_exit_prepare) .. kuap_assert_locked(); syscall_exit_to_user_mode(regs); <<< too early syscall_exit_to_user_mode_prepare(regs); <<< too early kuap_user_restore(regs); <<< already calls syscall_exit_prepare(), which can enable IRQs, replay a pending interrupt, and only then rfi. Those functions already restore KUAP immediately before rfi. Note that if we restore the user AMR too early like in the current code as shown from the code snippets above, then we get the following warning when CONFIG_PPC_KUAP_DEBUG is enabled: WARNING: arch/powerpc/include/asm/book3s/64/kup.h:293 at interrupt_exit_user_prepare+0x1a0/0x1c0 Hardware name: IBM pSeries (emulated by qemu) POWER10 (architected) TRAP: 0700 LR: c00000000000d8d4 CTR: c0000000021fe500 MSR: <SF,EE,ME,IR,DR,RI,LE> CR: 44000804 XER: 20040000 interrupt_exit_user_prepare+0x1a0/0x1c0 interrupt_return_srr_user+0x8/0x12c | |
| Title | powerpc: Do not restore KUAP in arch_exit_to_user_mode_prepare() | |
| First Time appeared |
Linux
Linux linux Kernel |
|
| CPEs | cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:* | |
| Vendors & Products |
Linux
Linux linux Kernel |
|
| References |
|
Projects
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MITRE
Status: PUBLISHED
Assigner: Linux
Published:
Updated: 2026-10-14T13:25:58.490Z
Reserved: 2026-09-25T10:25:14.346Z
Link: CVE-2026-98396
Vulnrichment
No data.
NVD
Status : Received
Published: 2026-10-14T14:17:50.530
Modified: 2026-10-14T14:17:50.530
Link: CVE-2026-98396
Redhat
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OpenCVE Enrichment
No data.
Weaknesses
No weakness.