On 09/20, Catalin Marinas wrote: > > On Fri, Sep 20, 2024 at 04:58:31PM +0800, Liao, Chang wrote: > > > > > > 在 2024/9/19 22:18, Oleg Nesterov 写道: > > > On 09/19, Liao Chang wrote: > > >> > > >> --- a/arch/arm64/kernel/probes/uprobes.c > > >> +++ b/arch/arm64/kernel/probes/uprobes.c > > >> @@ -17,12 +17,16 @@ void arch_uprobe_copy_ixol(struct page *page, > > >> unsigned long vaddr, > > >> void *xol_page_kaddr = kmap_atomic(page); > > >> void *dst = xol_page_kaddr + (vaddr & ~PAGE_MASK); > > >> > > >> + if (!memcmp(dst, src, len)) > > >> + goto done; > > > > > > can't really comment, I know nothing about arm64... > > > > > > but don't we need to change __create_xol_area() > > > > > > - area->page = alloc_page(GFP_HIGHUSER); > > > + area->page = alloc_page(GFP_HIGHUSER | __GFP_ZERO); > > > > > > to avoid the false positives? > > > > Indeed, it would be safer. > > > > Could we tolerate these false positives? Even if the page are not reset > > to zero bits, if the existing bits are the same as the instruction being > > copied, it still can execute the correct instruction. > > Not if the I-cache has stale data. If alloc_page() returns a page with > some random data that resembles a valid instruction but there was never > a cache flush (sync_icache_aliases() on arm64), it's irrelevant whether > the compare (on the D-cache side) succeeds or not.
But shouldn't the page fault paths on arm64 flush I-cache ? If alloc_page() returns a page with some random data that resembles a valid instruction, user-space can't execute this instruction until special_mapping_fault() installs the page allocated in __create_xol_area(). Again, I know nothing about arm64/icache/etc, I am just curious and trying to understand... Oleg.
