Re: [PATCH] powerpc: Fix random segfault when freeing hugetlb range

[Christophe Leroy]

Le 02/09/2020 à 05:23, Aneesh Kumar K.V a écrit :
> Christophe Leroy <christophe.leroy@xxxxxxxxxx> writes:
>
> > The following random segfault is observed from time to time with
> > map_hugetlb selftest:
> >
> > root@localhost:~# ./map_hugetlb 1 19
> > 524288 kB hugepages
> > Mapping 1 Mbytes
> > Segmentation fault
> >
> > [   31.219972] map_hugetlb[365]: segfault (11) at 117 nip 77974f8c lr 779a6834 code 1 in ld-2.23.so[77966000+21000]
> > [   31.220192] map_hugetlb[365]: code: 9421ffc0 480318d1 93410028 90010044 9361002c 93810030 93a10034 93c10038
> > [   31.220307] map_hugetlb[365]: code: 93e1003c 93210024 8123007c 81430038 <80e90004> 814a0004 7f443a14 813a0004
> > [   31.221911] BUG: Bad rss-counter state mm:(ptrval) type:MM_FILEPAGES val:33
> > [   31.229362] BUG: Bad rss-counter state mm:(ptrval) type:MM_ANONPAGES val:5
> >
> > This fault is due to hugetlb_free_pgd_range() freeing page tables
> > that are also used by regular pages.
> >
> > As explain in the comment at the beginning of
> > hugetlb_free_pgd_range(), the verification done in free_pgd_range()
> > on floor and ceiling is not done here, which means
> > hugetlb_free_pte_range() can free outside the expected range.
> >
> > As the verification cannot be done in hugetlb_free_pgd_range(), it
> > must be done in hugetlb_free_pte_range().
>
> Reviewed-by: Aneesh Kumar K.V <aneesh.kumar@xxxxxxxxxxxxx>
>
> > Fixes: b250c8c08c79 ("powerpc/8xx: Manage 512k huge pages as standard pages.")
> > Cc: stable@xxxxxxxxxxxxxxx
> > Signed-off-by: Christophe Leroy <christophe.leroy@xxxxxxxxxx>
> > ---
> >   arch/powerpc/mm/hugetlbpage.c | 18 ++++++++++++++++--
> >   1 file changed, 16 insertions(+), 2 deletions(-)
> >
> > diff --git a/arch/powerpc/mm/hugetlbpage.c b/arch/powerpc/mm/hugetlbpage.c
> > index 26292544630f..e7ae2a2c4545 100644
> > --- a/arch/powerpc/mm/hugetlbpage.c
> > +++ b/arch/powerpc/mm/hugetlbpage.c
> > @@ -330,10 +330,24 @@ static void free_hugepd_range(struct mmu_gather *tlb, hugepd_t *hpdp, int pdshif
> >   				 get_hugepd_cache_index(pdshift - shift));
> >   }
> >   
> > -static void hugetlb_free_pte_range(struct mmu_gather *tlb, pmd_t *pmd, unsigned long addr)
> > +static void hugetlb_free_pte_range(struct mmu_gather *tlb, pmd_t *pmd,
> > +				   unsigned long addr, unsigned long end,
> > +				   unsigned long floor, unsigned long ceiling)
> >   {
> > +	unsigned long start = addr;
> >   	pgtable_t token = pmd_pgtable(*pmd);
> >   
> > +	start &= PMD_MASK;
> > +	if (start < floor)
> > +		return;
> > +	if (ceiling) {
> > +		ceiling &= PMD_MASK;
> > +		if (!ceiling)
> > +			return;
> > +	}
> > +	if (end - 1 > ceiling - 1)
> > +		return;
> > +
>
> We do repeat that for pud/pmd/pte hugetlb_free_range. Can we consolidate
> that with comment explaining we are checking if the pgtable entry is
> mapping outside the range?

I was thinking about refactoring that into a helper and add all the 
necessary comments to explain what it does.

Will do that in a followup series if you are OK. This patch is a bug fix 
and also have to go through stable.

Christophe