| Message ID | 20200607155154.15928-1-julien@xen.org (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | [for-4.14] xen/arm: mm: Access a PT entry before the table is unmapped | expand |
> -----Original Message----- > From: Julien Grall <julien@xen.org> > Sent: 07 June 2020 16:52 > To: xen-devel@lists.xenproject.org > Cc: paul@xen.org; Julien Grall <jgrall@amazon.com>; Stefano Stabellini <sstabellini@kernel.org>; > Julien Grall <julien@xen.org>; Volodymyr Babchuk <Volodymyr_Babchuk@epam.com> > Subject: [PATCH for-4.14] xen/arm: mm: Access a PT entry before the table is unmapped > > From: Julien Grall <jgrall@amazon.com> > > xen_pt_next_level() will retrieve the MFN from the entry right after the > page-table has been unmapped. > > After calling xen_unmap_table(), there is no guarantee the mapping will > still be valid. Depending on the implementation, this may result to a > data abort in Xen. > > Re-order the code to retrieve the MFN before the table is unmapped. > > Fixes: 53abb9a1dcd9 ("xen/arm: mm: Rework Xen page-tables walk during update") > Signed-off-by: Julien Grall <jgrall@amazon.com> > Release-acked-by: Paul Durrant <paul@xen.org> > --- > > I spotted this issue while reworking how page-tables are mapped on Arm64 > during early boot. However the problem should be already there on Arm32. > > I am actually quite amazed we haven't seen any crash on Arm32 because > there is no direct map. So the mapping will not exist after calling > xen_unmap_table(). > > I suspect this works because unmap_domain_page() doesn't flush the > TLBs. So the TLB still likely have the entry cached (joy!). > > This patch is a candidate for Xen 4.14 and should also be backported. > --- > xen/arch/arm/mm.c | 5 ++++- > 1 file changed, 4 insertions(+), 1 deletion(-) > > diff --git a/xen/arch/arm/mm.c b/xen/arch/arm/mm.c > index 1b14f4934570..9e2ff7c8005d 100644 > --- a/xen/arch/arm/mm.c > +++ b/xen/arch/arm/mm.c > @@ -1036,6 +1036,7 @@ static int xen_pt_next_level(bool read_only, unsigned int level, > { > lpae_t *entry; > int ret; > + mfn_t mfn; > > entry = *table + offset; > > @@ -1053,8 +1054,10 @@ static int xen_pt_next_level(bool read_only, unsigned int level, > if ( lpae_is_mapping(*entry, level) ) > return XEN_TABLE_SUPER_PAGE; > > + mfn = lpae_get_mfn(*entry); > + > xen_unmap_table(*table); > - *table = xen_map_table(lpae_get_mfn(*entry)); > + *table = xen_map_table(mfn); > > return XEN_TABLE_NORMAL_PAGE; > } > -- > 2.17.1
On Sun, 7 Jun 2020, Julien Grall wrote: > From: Julien Grall <jgrall@amazon.com> > > xen_pt_next_level() will retrieve the MFN from the entry right after the > page-table has been unmapped. > > After calling xen_unmap_table(), there is no guarantee the mapping will > still be valid. Depending on the implementation, this may result to a > data abort in Xen. > > Re-order the code to retrieve the MFN before the table is unmapped. > > Fixes: 53abb9a1dcd9 ("xen/arm: mm: Rework Xen page-tables walk during update") > Signed-off-by: Julien Grall <jgrall@amazon.com> > > --- > > I spotted this issue while reworking how page-tables are mapped on Arm64 > during early boot. However the problem should be already there on Arm32. > > I am actually quite amazed we haven't seen any crash on Arm32 because > there is no direct map. So the mapping will not exist after calling > xen_unmap_table(). > > I suspect this works because unmap_domain_page() doesn't flush the > TLBs. So the TLB still likely have the entry cached (joy!). \0/ Damn! Reviewed-by: Stefano Stabellini <sstabellini@kernel.org> > This patch is a candidate for Xen 4.14 and should also be backported. > --- > xen/arch/arm/mm.c | 5 ++++- > 1 file changed, 4 insertions(+), 1 deletion(-) > > diff --git a/xen/arch/arm/mm.c b/xen/arch/arm/mm.c > index 1b14f4934570..9e2ff7c8005d 100644 > --- a/xen/arch/arm/mm.c > +++ b/xen/arch/arm/mm.c > @@ -1036,6 +1036,7 @@ static int xen_pt_next_level(bool read_only, unsigned int level, > { > lpae_t *entry; > int ret; > + mfn_t mfn; > > entry = *table + offset; > > @@ -1053,8 +1054,10 @@ static int xen_pt_next_level(bool read_only, unsigned int level, > if ( lpae_is_mapping(*entry, level) ) > return XEN_TABLE_SUPER_PAGE; > > + mfn = lpae_get_mfn(*entry); > + > xen_unmap_table(*table); > - *table = xen_map_table(lpae_get_mfn(*entry)); > + *table = xen_map_table(mfn); > > return XEN_TABLE_NORMAL_PAGE; > } > -- > 2.17.1 >
diff --git a/xen/arch/arm/mm.c b/xen/arch/arm/mm.c index 1b14f4934570..9e2ff7c8005d 100644 --- a/xen/arch/arm/mm.c +++ b/xen/arch/arm/mm.c @@ -1036,6 +1036,7 @@ static int xen_pt_next_level(bool read_only, unsigned int level, { lpae_t *entry; int ret; + mfn_t mfn; entry = *table + offset; @@ -1053,8 +1054,10 @@ static int xen_pt_next_level(bool read_only, unsigned int level, if ( lpae_is_mapping(*entry, level) ) return XEN_TABLE_SUPER_PAGE; + mfn = lpae_get_mfn(*entry); + xen_unmap_table(*table); - *table = xen_map_table(lpae_get_mfn(*entry)); + *table = xen_map_table(mfn); return XEN_TABLE_NORMAL_PAGE; }