Messages in this thread | | | Date | Mon, 7 Nov 2022 06:50:51 -0800 | Subject | Re: [PATCHv11 05/16] x86/uaccess: Provide untagged_addr() and remove tags before address check | From | Andy Lutomirski <> |
| |
On 10/24/22 17:17, Kirill A. Shutemov wrote: > untagged_addr() is a helper used by the core-mm to strip tag bits and > get the address to the canonical shape. In only handles userspace > addresses. The untagging mask is stored in mmu_context and will be set > on enabling LAM for the process. > > The tags must not be included into check whether it's okay to access the > userspace address. > > Strip tags in access_ok(). > > get_user() and put_user() don't use access_ok(), but check access > against TASK_SIZE directly in assembly. Strip tags, before calling into > the assembly helper. > > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com> > Tested-by: Alexander Potapenko <glider@google.com> > Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org> > --- > arch/x86/include/asm/mmu.h | 3 +++ > arch/x86/include/asm/mmu_context.h | 11 ++++++++ > arch/x86/include/asm/uaccess.h | 42 +++++++++++++++++++++++++++--- > arch/x86/kernel/process.c | 3 +++ > 4 files changed, 56 insertions(+), 3 deletions(-) > > diff --git a/arch/x86/include/asm/mmu.h b/arch/x86/include/asm/mmu.h > index 002889ca8978..2fdb390040b5 100644 > --- a/arch/x86/include/asm/mmu.h > +++ b/arch/x86/include/asm/mmu.h > @@ -43,6 +43,9 @@ typedef struct { > > /* Active LAM mode: X86_CR3_LAM_U48 or X86_CR3_LAM_U57 or 0 (disabled) */ > unsigned long lam_cr3_mask; > + > + /* Significant bits of the virtual address. Excludes tag bits. */ > + u64 untag_mask; > #endif > > struct mutex lock; > diff --git a/arch/x86/include/asm/mmu_context.h b/arch/x86/include/asm/mmu_context.h > index 69c943b2ae90..5bd3d46685dc 100644 > --- a/arch/x86/include/asm/mmu_context.h > +++ b/arch/x86/include/asm/mmu_context.h > @@ -100,6 +100,12 @@ static inline unsigned long mm_lam_cr3_mask(struct mm_struct *mm) > static inline void dup_lam(struct mm_struct *oldmm, struct mm_struct *mm) > { > mm->context.lam_cr3_mask = oldmm->context.lam_cr3_mask; > + mm->context.untag_mask = oldmm->context.untag_mask; > +} > + > +static inline void mm_reset_untag_mask(struct mm_struct *mm) > +{ > + mm->context.untag_mask = -1UL; > } > > #else > @@ -112,6 +118,10 @@ static inline unsigned long mm_lam_cr3_mask(struct mm_struct *mm) > static inline void dup_lam(struct mm_struct *oldmm, struct mm_struct *mm) > { > } > + > +static inline void mm_reset_untag_mask(struct mm_struct *mm) > +{ > +} > #endif > > #define enter_lazy_tlb enter_lazy_tlb > @@ -138,6 +148,7 @@ static inline int init_new_context(struct task_struct *tsk, > mm->context.execute_only_pkey = -1; > } > #endif > + mm_reset_untag_mask(mm); > init_new_context_ldt(mm); > return 0; > } > diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h > index 8bc614cfe21b..c6062c07ccd2 100644 > --- a/arch/x86/include/asm/uaccess.h > +++ b/arch/x86/include/asm/uaccess.h > @@ -7,6 +7,7 @@ > #include <linux/compiler.h> > #include <linux/instrumented.h> > #include <linux/kasan-checks.h> > +#include <linux/mm_types.h> > #include <linux/string.h> > #include <asm/asm.h> > #include <asm/page.h> > @@ -21,6 +22,30 @@ static inline bool pagefault_disabled(void); > # define WARN_ON_IN_IRQ() > #endif > > +#ifdef CONFIG_X86_64 > +/* > + * Mask out tag bits from the address. > + * > + * Magic with the 'sign' allows to untag userspace pointer without any branches > + * while leaving kernel addresses intact. > + */ > +#define untagged_addr(mm, addr) ({ \ > + u64 __addr = (__force u64)(addr); \ > + s64 sign = (s64)__addr >> 63; \ > + __addr &= (mm)->context.untag_mask | sign; \ > + (__force __typeof__(addr))__addr; \ > +}) > +
I think this implementation is correct, but I'm wondering if there are any callers of untagged_addr that actually need to preserve kernel addresses. Are there? (There certainly *were* back when we had set_fs().)
I'm also mildly uneasy about a potential edge case. Naively, one would expect:
untagged_addr(current->mm, addr) + size == untagged_addr(current->mm, addr + size)
at least for an address that is valid enough to be potentially dereferenced. This isn't true any more for size that overflows into the tag bit range.
I *think* we're okay though -- __access_ok requires that addr <= limit - size, so any range that overflows into tag bits will be rejected even if the entire range consists of valid (tagged) user addresses.
So:
Acked-by: Andy Lutomirski <luto@kernel.org>
| |