Re: [PATCH v15 2/4] syscall user dispatch: untag selector addresses before access_ok

[Catalin Marinas <catalin.marinas@arm.com>]

On Tue, Apr 04, 2023 at 12:45:06PM +0200, Oleg Nesterov wrote:
> doesn't this mean that access_ok() on arm64 could use
> untagged_addr(addr) unconditionally without any security risk?

Yes, from the security perspective, but there are ABI implications.

Currently untagged_addr() in access_ok() is conditional on the user
process enabling the tagged address ABI (prctl() that sets a TIF flag).
The reason we did not enable this by default was a slight fear of
breaking the ABI since tagged pointers were not allowed at the syscall
boundary. It turned out that the fear was justified since the
unconditional untagged_addr() in brk() broke user space (see commit
dcde237319e6 "mm: Avoid creating virtual address aliases in
brk()/mmap()/mremap()"; the user was doing an sbrk(PY_SSIZE_T_MAX) and
bits 56 and higher were ignored by the kernel).

I'd be ok with untagging the address unconditionally in the arm64
access_ok() introduce another unaliased_access_ok() (I'm not good at
naming functions) that preserves the non-tagged behaviour and we use it
in brk/mmap/mremap().

This may actually be a good idea as an additional fix. If an application
enables the tagged address ABI we still have the address aliasing issue
for brk(). We get away with this since glibc, if MTE is enabled, stops
using brk() for heap in favour of mmap(PROT_MTE). But one may use hwasan
without full MTE and it would have a similar issue.

-- 
Catalin