> On April 15, 2020 4:12 AM Christophe Leroy <christophe.le...@c-s.fr> wrote: > > > Le 15/04/2020 à 07:16, Christopher M Riedl a écrit : > >> On March 26, 2020 9:42 AM Christophe Leroy <christophe.le...@c-s.fr> wrote: > >> > >> > >> This patch fixes the RFC series identified below. > >> It fixes three points: > >> - Failure with CONFIG_PPC_KUAP > >> - Failure to write do to lack of DIRTY bit set on the 8xx > >> - Inadequaly complex WARN post verification > >> > >> However, it has an impact on the CPU load. Here is the time > >> needed on an 8xx to run the ftrace selftests without and > >> with this series: > >> - Without CONFIG_STRICT_KERNEL_RWX ==> 38 seconds > >> - With CONFIG_STRICT_KERNEL_RWX ==> 40 seconds > >> - With CONFIG_STRICT_KERNEL_RWX + this series ==> 43 seconds > >> > >> Link: https://patchwork.ozlabs.org/project/linuxppc-dev/list/?series=166003 > >> Signed-off-by: Christophe Leroy <christophe.le...@c-s.fr> > >> --- > >> arch/powerpc/lib/code-patching.c | 5 ++++- > >> 1 file changed, 4 insertions(+), 1 deletion(-) > >> > >> diff --git a/arch/powerpc/lib/code-patching.c > >> b/arch/powerpc/lib/code-patching.c > >> index f156132e8975..4ccff427592e 100644 > >> --- a/arch/powerpc/lib/code-patching.c > >> +++ b/arch/powerpc/lib/code-patching.c > >> @@ -97,6 +97,7 @@ static int map_patch(const void *addr, struct > >> patch_mapping *patch_mapping) > >> } > >> > >> pte = mk_pte(page, pgprot); > >> + pte = pte_mkdirty(pte); > >> set_pte_at(patching_mm, patching_addr, ptep, pte); > >> > >> init_temp_mm(&patch_mapping->temp_mm, patching_mm); > >> @@ -168,7 +169,9 @@ static int do_patch_instruction(unsigned int *addr, > >> unsigned int instr) > >> (offset_in_page((unsigned long)addr) / > >> sizeof(unsigned int)); > >> > >> + allow_write_to_user(patch_addr, sizeof(instr)); > >> __patch_instruction(addr, instr, patch_addr); > >> + prevent_write_to_user(patch_addr, sizeof(instr)); > >> > > > > On radix we can map the page with PAGE_KERNEL protection which ends up > > setting EAA[0] in the radix PTE. This means the KUAP (AMR) protection is > > ignored (ISA v3.0b Fig. 35) since we are accessing the page from MSR[PR]=0. > > > > Can we employ a similar approach on the 8xx? I would prefer *not* to wrap > > the __patch_instruction() with the allow_/prevent_write_to_user() KUAP > > things > > because this is a temporary kernel mapping which really isn't userspace in > > the usual sense. > > On the 8xx, that's pretty different. > > The PTE doesn't control whether a page is user page or a kernel page. > The only thing that is set in the PTE is whether a page is linked to a > given PID or not. > PAGE_KERNEL tells that the page can be addressed with any PID. > > The user access right is given by a kind of zone, which is in the PGD > entry. Every pages above PAGE_OFFSET are defined as belonging to zone 0. > Every pages below PAGE_OFFSET are defined as belonging to zone 1. > > By default, zone 0 can only be accessed by kernel, and zone 1 can only > be accessed by user. When kernel wants to access zone 1, it temporarily > changes properties of zone 1 to allow both kernel and user accesses. > > So, if your mapping is below PAGE_OFFSET, it is in zone 1 and kernel > must unlock it to access it. > > > And this is more or less the same on hash/32. This is managed by segment > registers. One segment register corresponds to a 256Mbytes area. Every > pages below PAGE_OFFSET can only be read by default by kernel. Only user > can write if the PTE allows it. When the kernel needs to write at an > address below PAGE_OFFSET, it must change the segment properties in the > corresponding segment register. > > So, for both cases, if we want to have it local to a task while still > allowing kernel access, it means we have to define a new special area > between TASK_SIZE and PAGE_OFFSET which belongs to kernel zone. > > That looks complex to me for a small benefit, especially as 8xx is not > SMP and neither are most of the hash/32 targets. >
Agreed. So I guess the solution is to differentiate between radix/non-radix and use PAGE_SHARED for non-radix along with the KUAP functions when KUAP is enabled. Hmm, I need to think about this some more, especially if it's acceptable to temporarily map kernel text as PAGE_SHARED for patching. Do you see any obvious problems on 8xx and hash/32 w/ using PAGE_SHARED? I don't necessarily want to drop the local mm patching idea for non-radix platforms since that means we would have to maintain two implementations. > Christophe
