Messages in this thread | | | Subject | Re: [RFC PATCH] powerpc/lib: Fixing use a temporary mm for code patching | From | Christophe Leroy <> | Date | Sat, 18 Apr 2020 12:27:14 +0200 |
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Le 15/04/2020 à 18:22, Christopher M Riedl a écrit : >> On April 15, 2020 4:12 AM Christophe Leroy <christophe.leroy@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.leroy@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.leroy@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?
No it shouldn't be a problem AFAICS, except maybe the CPU overhead it brings as I mentioned previously (ftrace selftests going from 40 to 43 seconds ie 8% overhead.
> > 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. >
What's the problem with RADIX, why can't PAGE_SHARED be used on radix ?
Christophe
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