Re: [PATCH v4 00/10] powerpc: Implement masked user access

[Madhavan Srinivasan <maddy@linux.ibm.com>]

On 11/6/25 5:01 PM, Christophe Leroy wrote:
> This is a rebase on top of commit 6ec821f050e2 (tag: core-scoped-uaccess)
> from tip tree.
> 
> Thomas, Peter, could you please take non-powerpc patches (1, 2, 3)
> in tip tree for v6.19, then Maddy will take powerpc patches (4-10)
> into powerpc-next for v6.20.

Thomas/Peter,

If you can please take non-powerpc patches in this series
in v6.19, I will park the rest of the series (4-10)
to the v6.20 merge.

Maddy

> 
> Masked user access avoids the address/size verification by access_ok().
> Allthough its main purpose is to skip the speculation in the
> verification of user address and size hence avoid the need of spec
> mitigation, it also has the advantage to reduce the amount of
> instructions needed so it also benefits to platforms that don't
> need speculation mitigation, especially when the size of the copy is
> not know at build time.
> 
> Patches 1,2,4 are cleaning up some redundant barrier_nospec()
> introduced by commit 74e19ef0ff80 ("uaccess: Add speculation barrier
> to copy_from_user()"). To do that, a speculation barrier is added to
> copy_from_user_iter() so that the barrier in powerpc raw_copy_from_user()
> which is redundant with the one in copy_from_user() can be removed. To
> avoid impacting x86, copy_from_user_iter() is first converted to using
> masked user access.
> 
> Patch 3 replaces wrong calls to masked_user_access_begin() with calls
> to masked_user_read_access_begin() and masked_user_write_access_begin()
> to match with user_read_access_end() and user_write_access_end().
> 
> Patches 5,6,7 are cleaning up powerpc uaccess functions.
> 
> Patches 8 and 9 prepare powerpc/32 for the necessary gap at the top
> of userspace.
> 
> Last patch implements masked user access.
> 
> Changes in v4:
> - Rebased on top of commit 6ec821f050e2 (tag: core-scoped-uaccess) from tip tree
> - Patch 3: Simplified as masked_user_read_access_begin() and masked_user_write_access_begin() are already there.
> - Patch 10: Simplified mask_user_address_simple() as suggested by Gabriel.
> 
> Changes in v3:
> - Rebased on top of v6.18-rc1
> - Patch 3: Impact on recently modified net/core/scm.c
> - Patch 10: Rewrite mask_user_address_simple() for a smaller result on powerpc64, suggested by Gabriel
> 
> Changes in v2:
> - Converted copy_from_user_iter() to using masked user access.
> - Cleaned up powerpc uaccess function to minimise code duplication
> when adding masked user access
> - Automated TASK_SIZE calculation to minimise use of BUILD_BUG_ON()
> - Tried to make some commit messages more clean based on feedback from
> version 1 of the series.
> 
> Christophe Leroy (10):
>   iter: Avoid barrier_nospec() in copy_from_user_iter()
>   uaccess: Add speculation barrier to copy_from_user_iter()
>   uaccess: Use masked_user_{read/write}_access_begin when required
>   powerpc/uaccess: Move barrier_nospec() out of
>     allow_read_{from/write}_user()
>   powerpc/uaccess: Remove unused size and from parameters from
>     allow_access_user()
>   powerpc/uaccess: Remove
>     {allow/prevent}_{read/write/read_write}_{from/to/}_user()
>   powerpc/uaccess: Refactor user_{read/write/}_access_begin()
>   powerpc/32s: Fix segments setup when TASK_SIZE is not a multiple of
>     256M
>   powerpc/32: Automatically adapt TASK_SIZE based on constraints
>   powerpc/uaccess: Implement masked user access
> 
>  arch/powerpc/Kconfig                          |   3 +-
>  arch/powerpc/include/asm/barrier.h            |   2 +-
>  arch/powerpc/include/asm/book3s/32/kup.h      |   3 +-
>  arch/powerpc/include/asm/book3s/32/mmu-hash.h |   5 +-
>  arch/powerpc/include/asm/book3s/32/pgtable.h  |   4 -
>  arch/powerpc/include/asm/book3s/64/kup.h      |   6 +-
>  arch/powerpc/include/asm/kup.h                |  52 +------
>  arch/powerpc/include/asm/nohash/32/kup-8xx.h  |   3 +-
>  arch/powerpc/include/asm/nohash/32/mmu-8xx.h  |   4 -
>  arch/powerpc/include/asm/nohash/kup-booke.h   |   3 +-
>  arch/powerpc/include/asm/task_size_32.h       |  28 +++-
>  arch/powerpc/include/asm/uaccess.h            | 132 +++++++++++++-----
>  arch/powerpc/kernel/asm-offsets.c             |   2 +-
>  arch/powerpc/kernel/head_book3s_32.S          |   6 +-
>  arch/powerpc/mm/book3s32/mmu.c                |   4 +-
>  arch/powerpc/mm/mem.c                         |   2 -
>  arch/powerpc/mm/nohash/8xx.c                  |   2 -
>  arch/powerpc/mm/ptdump/segment_regs.c         |   2 +-
>  lib/iov_iter.c                                |  22 ++-
>  lib/strncpy_from_user.c                       |   2 +-
>  lib/strnlen_user.c                            |   2 +-
>  net/core/scm.c                                |   2 +-
>  22 files changed, 161 insertions(+), 130 deletions(-)
>