From: "Russell King (Oracle)" <rmk+kernel@armlinux.org.uk> mainline inclusion from mainline-v6.19-rc1 commit 7733bc7d299d682f2723dc38fc7f370b9bf973e9 category: bugfix bugzilla: https://gitee.com/openeuler/kernel/issues/IDD7MD Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?i... -------------------------------- Zizhi Wo reports: "During the execution of hash_name()->load_unaligned_zeropad(), a potential memory access beyond the PAGE boundary may occur. For example, when the filename length is near the PAGE_SIZE boundary. This triggers a page fault, which leads to a call to do_page_fault()->mmap_read_trylock(). If we can't acquire the lock, we have to fall back to the mmap_read_lock() path, which calls might_sleep(). This breaks RCU semantics because path lookup occurs under an RCU read-side critical section." This is seen with CONFIG_DEBUG_ATOMIC_SLEEP=y and CONFIG_KFENCE=y. Kernel addresses (with the exception of the vectors/kuser helper page) do not have VMAs associated with them. If the vectors/kuser helper page faults, then there are two possibilities: 1. if the fault happened while in kernel mode, then we're basically dead, because the CPU won't be able to vector through this page to handle the fault. 2. if the fault happened while in user mode, that means the page was protected from user access, and we want to fault anyway. Thus, we can handle kernel addresses from any context entirely separately without going anywhere near the mmap lock. This gives us an entirely non-sleeping path for all kernel mode kernel address faults. As we handle the kernel address faults before interrupts are enabled, this change has the side effect of improving the branch predictor hardening, but does not completely solve the issue. Reported-by: Zizhi Wo <wozizhi@huaweicloud.com> Reported-by: Xie Yuanbin <xieyuanbin1@huawei.com> Link: https://lore.kernel.org/r/20251126090505.3057219-1-wozizhi@huaweicloud.com Reviewed-by: Xie Yuanbin <xieyuanbin1@huawei.com> Tested-by: Xie Yuanbin <xieyuanbin1@huawei.com> Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk> Conflicts: arch/arm/mm/fault.c [A simple context conflict, unrelated to this patch.] Fixes: bfcfaa77bdf0 ("vfs: use 'unsigned long' accesses for dcache name comparison and hashing") Signed-off-by: Zizhi Wo <wozizhi@huawei.com> --- arch/arm/mm/fault.c | 35 +++++++++++++++++++++++++++++++++++ 1 file changed, 35 insertions(+) diff --git a/arch/arm/mm/fault.c b/arch/arm/mm/fault.c index 4c0ee81befb1..47eecdf29a83 100644 --- a/arch/arm/mm/fault.c +++ b/arch/arm/mm/fault.c @@ -244,6 +244,35 @@ void do_bad_area(unsigned long addr, unsigned int fsr, struct pt_regs *regs) #define VM_FAULT_BADMAP ((__force vm_fault_t)0x010000) #define VM_FAULT_BADACCESS ((__force vm_fault_t)0x020000) +static int __kprobes +do_kernel_address_page_fault(struct mm_struct *mm, unsigned long addr, + unsigned int fsr, struct pt_regs *regs) +{ + if (user_mode(regs)) { + /* + * Fault from user mode for a kernel space address. User mode + * should not be faulting in kernel space, which includes the + * vector/khelper page. Send a SIGSEGV. + */ + __do_user_fault(addr, fsr, SIGSEGV, SEGV_MAPERR, regs); + } else { + /* + * Fault from kernel mode. Enable interrupts if they were + * enabled in the parent context. Section (upper page table) + * translation faults are handled via do_translation_fault(), + * so we will only get here for a non-present kernel space + * PTE or PTE permission fault. This may happen in exceptional + * circumstances and need the fixup tables to be walked. + */ + if (interrupts_enabled(regs)) + local_irq_enable(); + + __do_kernel_fault(mm, addr, fsr, regs); + } + + return 0; +} + static int __kprobes do_page_fault(unsigned long addr, unsigned int fsr, struct pt_regs *regs) { @@ -257,6 +286,12 @@ do_page_fault(unsigned long addr, unsigned int fsr, struct pt_regs *regs) if (kprobe_page_fault(regs, fsr)) return 0; + /* + * Handle kernel addresses faults separately, which avoids touching + * the mmap lock from contexts that are not able to sleep. + */ + if (addr >= TASK_SIZE) + return do_kernel_address_page_fault(mm, addr, fsr, regs); /* Enable interrupts if they were enabled in the parent context. */ if (interrupts_enabled(regs)) -- 2.39.2