Re: [PATCH 2/2] kasan, arm64: fix pointer tags in KASAN reports

On Fri, Jan 15, 2021 at 6:06 PM Catalin Marinas [off-list ref] wrote:

On Fri, Jan 15, 2021 at 06:00:36PM +0100, Andrey Konovalov wrote:

quoted

On Fri, Jan 15, 2021 at 5:56 PM Catalin Marinas [off-list ref] wrote:

quoted

On Fri, Jan 15, 2021 at 05:30:40PM +0100, Andrey Konovalov wrote:

quoted

On Wed, Jan 13, 2021 at 5:54 PM Catalin Marinas [off-list ref] wrote:

quoted

On Wed, Jan 13, 2021 at 05:03:30PM +0100, Andrey Konovalov wrote:

quoted

As of the "arm64: expose FAR_EL1 tag bits in siginfo" patch, the address
that is passed to report_tag_fault has pointer tags in the format of 0x0X,
while KASAN uses 0xFX format (note the difference in the top 4 bits).

Fix up the pointer tag before calling kasan_report.

Link: https://linux-review.googlesource.com/id/I9ced973866036d8679e8f4ae325de547eb969649
Fixes: dceec3ff7807 ("arm64: expose FAR_EL1 tag bits in siginfo")
Fixes: 4291e9ee6189 ("kasan, arm64: print report from tag fault handler")
Signed-off-by: Andrey Konovalov <redacted>
---
 arch/arm64/mm/fault.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
index 3c40da479899..a218f6f2fdc8 100644
--- a/arch/arm64/mm/fault.c
+++ b/arch/arm64/mm/fault.c

@@ -304,6 +304,8 @@ static void report_tag_fault(unsigned long addr, unsigned int esr,
 {
      bool is_write  = ((esr & ESR_ELx_WNR) >> ESR_ELx_WNR_SHIFT) != 0;

+     /* The format of KASAN tags is 0xF<x>. */
+     addr |= (0xF0UL << MTE_TAG_SHIFT);

Ah, I see, that top 4 bits are zeroed by do_tag_check_fault(). When this
was added, the only tag faults were generated for user addresses.

Anyway, I'd rather fix it in there based on bit 55, something like (only
compile-tested):

diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
index 3c40da479899..2b71079d2d32 100644
--- a/arch/arm64/mm/fault.c
+++ b/arch/arm64/mm/fault.c

@@ -709,10 +709,11 @@ static int do_tag_check_fault(unsigned long far, unsigned int esr,
                              struct pt_regs *regs)
 {
        /*
-        * The architecture specifies that bits 63:60 of FAR_EL1 are UNKNOWN for tag
-        * check faults. Mask them out now so that userspace doesn't see them.
+        * The architecture specifies that bits 63:60 of FAR_EL1 are UNKNOWN
+        * for tag check faults. Set them to the corresponding bits in the
+        * untagged address.
         */
-       far &= (1UL << 60) - 1;
+       far = (untagged_addr(far) & ~MTE_TAG_MASK) | (far & MTE_TAG_MASK) ;
        do_bad_area(far, esr, regs);
        return 0;
 }

BTW, we can do "untagged_addr(far) | (far & MTE_TAG_MASK)" here, as
untagged_addr() doesn't change kernel pointers.

untagged_addr() does change tagged kernel pointers, it sign-extends from
bit 55. So the top byte becomes 0xff and you can no longer or the tag
bits in.

That's __untagged_addr(), untagged_addr() keeps the bits for kernel
pointers as of  597399d0cb91.

Ah, you are right. In this case I think we should use __untagged_addr()
above. Even if the tag check fault happened on a kernel address, bits
63:60 are still unknown.

Yeah, I keep forgetting about [__]untagged_addr() too. Maybe we need
better names? Like untagged_addr() and untagged_addr_ttbr0()?

Anyway, I'll do the explicit calculation with __untagged_addr() in the
next version.

Thanks!

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