+ Dan and smatch@xxxxxxxxxxxxxxx Hi Andrew, I am adding Dan to this thread since he is the smatch maintainer, and the smatch@xxxxxxxxxxxxxxx list. @Dan and @smatch@xxxxxxxxxxxxxxx: a reference to the beginning of this thread can be found at [1]. [1] https://lkml.org/lkml/2019/6/19/376 On 6/27/19 2:18 PM, Andrew Murray wrote: > On Wed, Jun 26, 2019 at 06:45:03PM +0100, Catalin Marinas wrote: >> Hi Andrew, >> >> Cc'ing Luc (sparse maintainer) who's been involved in the past >> discussions around static checking of user pointers: >> >> https://lore.kernel.org/linux-arm-kernel/20180905190316.a34yycthgbamx2t3@ltop.local/ >> >> So I think the difference here from the previous approach is that we >> explicitly mark functions that cannot take tagged addresses (like >> find_vma()) and identify the callers. > > Indeed. > > >> >> More comments below: >> >> On Wed, Jun 19, 2019 at 01:16:20PM +0100, Andrew Murray wrote: >>> The proposed introduction of a relaxed ARM64 ABI [1] will allow tagged memory >>> addresses to be passed through the user-kernel syscall ABI boundary. Tagged >>> memory addresses are those which contain a non-zero top byte (the hardware >>> has always ignored this top byte due to TCR_EL1.TBI0) and may be useful >>> for features such as HWASan. >>> >>> To permit this relaxation a proposed patchset [2] strips the top byte (tag) >>> from user provided memory addresses prior to use in kernel functions which >>> require untagged addresses (for example comparasion/arithmetic of addresses). >>> The author of this patchset relied on a variety of techniques [2] (such as >>> grep, BUG_ON, sparse etc) to identify as many instances of possible where >>> tags need to be stipped. >>> >>> To support this effort and to catch future regressions (e.g. in new syscalls >>> or ioctls), I've devised an additional approach for detecting the use of >>> tagged addresses in functions that do not want them. This approach makes >>> use of Smatch [3] and is outlined in this RFC. Due to the ability of Smatch >>> to do flow analysis I believe we can annotate the kernel in fewer places >>> than a similar approach in sparse. >>> >>> I'm keen for feedback on the likely usefulness of this approach. >>> >>> We first add some new annotations that are exclusively consumed by Smatch: >>> >>> --- a/include/linux/compiler_types.h >>> +++ b/include/linux/compiler_types.h >>> @@ -19,6 +19,7 @@ >>> # define __cond_lock(x,c) ((c) ? ({ __acquire(x); 1; }) : 0) >>> # define __percpu __attribute__((noderef, address_space(3))) >>> # define __rcu __attribute__((noderef, address_space(4))) >>> +# define __untagged __attribute__((address_space(5))) >>> # define __private __attribute__((noderef)) >>> extern void __chk_user_ptr(const volatile void __user *); >>> extern void __chk_io_ptr(const volatile void __iomem *); >> [...] >>> --- a/mm/mmap.c >>> +++ b/mm/mmap.c >>> @@ -2224,7 +2224,7 @@ get_unmapped_area(struct file *file, unsigned long addr, unsigned long len, >>> EXPORT_SYMBOL(get_unmapped_area); >>> >>> /* Look up the first VMA which satisfies addr < vm_end, NULL if none. */ >>> -struct vm_area_struct *find_vma(struct mm_struct *mm, unsigned long addr) >>> +struct vm_area_struct *find_vma(struct mm_struct *mm, unsigned long __untagged addr) >>> { >>> struct rb_node *rb_node; >>> struct vm_area_struct *vma; >> [...] >>> This can be further improved - the problem here is that for a given function, >>> e.g. find_vma we look for callers where *any* of the parameters >>> passed to find_vma are tagged addresses from userspace - i.e. not *just* >>> the annotated parameter. This is also true for find_vma's callers' callers'. >>> This results in the call tree having false positives. >>> >>> It *is* possible to track parameters (e.g. find_vma arg 1 comes from arg 3 of >>> do_pages_stat_array etc), but this is limited as if functions modify the >>> data then the tracking is stopped (however this can be fixed). >> [...] >>> An example of a false positve is do_mlock. We untag the address and pass that >>> to apply_vma_lock_flags - however we also pass a length - because the length >>> came from userspace and could have the top bits set - it's flagged. However >>> with improved parameter tracking we can remove this false positive and similar. >> >> Could we track only the conversions from __user * that eventually end up >> as __untagged? (I'm not familiar with smatch, so not sure what it can >> do). > > I assume you mean 'that eventually end up as an argument annotated __untagged'? > > The warnings smatch currently produce relate to only the conversions you > mention - however further work is needed in smatch to improve the scripts that > retrospectively provide call traces (without false positives). > > >> We could assume that an unsigned long argument to a syscall is >> default __untagged, unless explicitly marked as __tagged. For example, >> sys_munmap() is allowed to take a tagged address. > > I'll give this some further thought. > > >> >>> Prior to smatch I attempted a similar approach with sparse - however it seemed >>> necessary to propogate the __untagged annotation in every function up the call tree, >>> and resulted in adding the __untagged annotation to functions that would never >>> get near user provided data. This leads to a littering of __untagged all over the >>> kernel which doesn't seem appealing. >> >> Indeed. We attempted this last year (see the above thread). >> >>> Smatch is more capable, however it almost >>> certainly won't pick up 100% of issues due to the difficulity of making flow >>> analysis understand everything a compiler can. >>> >>> Is it likely to be acceptable to use the __untagged annotation in user-path >>> functions that require untagged addresses across the kernel? >> >> If it helps with identifying missing untagged_addr() calls, I would say >> yes (as long as we keep them to a minimum). > > Thanks for the feedback. > > Andrew Murray > >> >>> [1] https://lkml.org/lkml/2019/6/13/534 >>> [2] https://patchwork.kernel.org/cover/10989517/ >>> [3] http://smatch.sourceforge.net/ >> >> -- >> Catalin -- Regards, Vincenzo
