6.15-stable review patch. If anyone has any objections, please let me know.
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From: Kevin Brodsky kevin.brodsky@arm.com
[ Upstream commit 22f3a4f6085951eff28bd1e44d3f388c1d9a5f44 ]
We do not currently issue an ISB after updating POR_EL0 when context-switching it, for instance. The rationale is that if the old value of POR_EL0 is more restrictive and causes a fault during uaccess, the access will be retried [1]. In other words, we are trading an ISB on every context-switching for the (unlikely) possibility of a spurious fault. We may also miss faults if the new value of POR_EL0 is more restrictive, but that's considered acceptable.
However, as things stand, a spurious Overlay fault results in uaccess failing right away since it causes fault_from_pkey() to return true. If an Overlay fault is reported, we therefore need to double check POR_EL0 against vma_pkey(vma) - this is what arch_vma_access_permitted() already does.
As it turns out, we already perform that explicit check if no Overlay fault is reported, and we need to keep that check (see comment added in fault_from_pkey()). Net result: the Overlay ISS2 bit isn't of much help to decide whether a pkey fault occurred.
Remove the check for the Overlay bit from fault_from_pkey() and add a comment to try and explain the situation. While at it, also add a comment to permission_overlay_switch() in case anyone gets surprised by the lack of ISB.
[1] https://lore.kernel.org/linux-arm-kernel/ZtYNGBrcE-j35fpw@arm.com/
Fixes: 160a8e13de6c ("arm64: context switch POR_EL0 register") Signed-off-by: Kevin Brodsky kevin.brodsky@arm.com Link: https://lore.kernel.org/r/20250619160042.2499290-2-kevin.brodsky@arm.com Signed-off-by: Will Deacon will@kernel.org Signed-off-by: Sasha Levin sashal@kernel.org --- arch/arm64/kernel/process.c | 5 +++++ arch/arm64/mm/fault.c | 30 +++++++++++++++++++++--------- 2 files changed, 26 insertions(+), 9 deletions(-)
diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c index 42faebb7b7123..4bc70205312e4 100644 --- a/arch/arm64/kernel/process.c +++ b/arch/arm64/kernel/process.c @@ -638,6 +638,11 @@ static void permission_overlay_switch(struct task_struct *next) current->thread.por_el0 = read_sysreg_s(SYS_POR_EL0); if (current->thread.por_el0 != next->thread.por_el0) { write_sysreg_s(next->thread.por_el0, SYS_POR_EL0); + /* + * No ISB required as we can tolerate spurious Overlay faults - + * the fault handler will check again based on the new value + * of POR_EL0. + */ } }
diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c index ec0a337891ddf..11eb8d1adc841 100644 --- a/arch/arm64/mm/fault.c +++ b/arch/arm64/mm/fault.c @@ -487,17 +487,29 @@ static void do_bad_area(unsigned long far, unsigned long esr, } }
-static bool fault_from_pkey(unsigned long esr, struct vm_area_struct *vma, - unsigned int mm_flags) +static bool fault_from_pkey(struct vm_area_struct *vma, unsigned int mm_flags) { - unsigned long iss2 = ESR_ELx_ISS2(esr); - if (!system_supports_poe()) return false;
- if (esr_fsc_is_permission_fault(esr) && (iss2 & ESR_ELx_Overlay)) - return true; - + /* + * We do not check whether an Overlay fault has occurred because we + * cannot make a decision based solely on its value: + * + * - If Overlay is set, a fault did occur due to POE, but it may be + * spurious in those cases where we update POR_EL0 without ISB (e.g. + * on context-switch). We would then need to manually check POR_EL0 + * against vma_pkey(vma), which is exactly what + * arch_vma_access_permitted() does. + * + * - If Overlay is not set, we may still need to report a pkey fault. + * This is the case if an access was made within a mapping but with no + * page mapped, and POR_EL0 forbids the access (according to + * vma_pkey()). Such access will result in a SIGSEGV regardless + * because core code checks arch_vma_access_permitted(), but in order + * to report the correct error code - SEGV_PKUERR - we must handle + * that case here. + */ return !arch_vma_access_permitted(vma, mm_flags & FAULT_FLAG_WRITE, mm_flags & FAULT_FLAG_INSTRUCTION, @@ -635,7 +647,7 @@ static int __kprobes do_page_fault(unsigned long far, unsigned long esr, goto bad_area; }
- if (fault_from_pkey(esr, vma, mm_flags)) { + if (fault_from_pkey(vma, mm_flags)) { pkey = vma_pkey(vma); vma_end_read(vma); fault = 0; @@ -679,7 +691,7 @@ static int __kprobes do_page_fault(unsigned long far, unsigned long esr, goto bad_area; }
- if (fault_from_pkey(esr, vma, mm_flags)) { + if (fault_from_pkey(vma, mm_flags)) { pkey = vma_pkey(vma); mmap_read_unlock(mm); fault = 0;