diff options
authorMichael Ellerman <mpe@ellerman.id.au>2017-06-22 16:52:51 +1000
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>2017-06-29 12:48:52 +0200
commitdb7130d63fd80256d448686a06a5154a8b9b4f62 (patch)
parent8fcb215c5426301fa6d49899028b7161dc189d88 (diff)
powerpc/slb: Force a full SLB flush when we insert for a bad EA
[Note this patch is not upstream. The bug fix was fixed differently in upstream prior to the bug being identified.] The SLB miss handler calls slb_allocate_realmode() in order to create an SLB entry for the faulting address. At the very start of that function we check that the faulting Effective Address (EA) is less than PGTABLE_RANGE (ignoring the region), ie. is it an address which could possibly fit in the virtual address space. For an EA which fails that test, we branch out of line (to label 8), but we still go on to create an SLB entry for the address. The SLB entry we create has a VSID of 0, which means it will never match anything in the hash table and so can't actually translate to a physical address. However that SLB entry will be inserted in the SLB, and so needs to be managed properly like any other SLB entry. In particular we need to insert the SLB entry in the SLB cache, so that it will be flushed when the process is descheduled. And that is where the bugs begin. The first bug is that slb_finish_load() uses cr7 to decide if it should insert the SLB entry into the SLB cache. When we come from the invalid EA case we don't set cr7, it just has some junk value from userspace. So we may or may not insert the SLB entry in the SLB cache. If we fail to insert it, we may then incorrectly leave it in the SLB when the process is descheduled. The second bug is that even if we do happen to add the entry to the SLB cache, we do not have enough bits in the SLB cache to remember the full ESID value for very large EAs. For example if a process branches to 0x788c545a18000000, that results in a 256MB SLB entry with an ESID of 0x788c545a1. But each entry in the SLB cache is only 32-bits, meaning we truncate the ESID to 0x88c545a1. This has the same effect as the first bug, we incorrectly leave the SLB entry in the SLB when the process is descheduled. When a process accesses an invalid EA it results in a SEGV signal being sent to the process, which typically results in the process being killed. Process death isn't instantaneous however, the process may catch the SEGV signal and continue somehow, or the kernel may start writing a core dump for the process, either of which means it's possible for the process to be preempted while its processing the SEGV but before it's been killed. If that happens, when the process is scheduled back onto the CPU we will allocate a new SLB entry for the NIP, which will insert a second entry into the SLB for the bad EA. Because we never flushed the original entry, due to either bug one or two, we now have two SLB entries that match the same EA. If another access is made to that EA, either by the process continuing after catching the SEGV, or by a second process accessing the same bad EA on the same CPU, we will trigger an SLB multi-hit machine check exception. This has been observed happening in the wild. The fix is when we hit the invalid EA case, we mark the SLB cache as being full. This causes us to not insert the truncated ESID into the SLB cache, and means when the process is switched out we will flush the entire SLB. Note that this works both for the original fault and for a subsequent call to slb_allocate_realmode() from switch_slb(). Because we mark the SLB cache as full, it doesn't really matter what value is in cr7, but rather than leaving it as something random we set it to indicate the address was a kernel address. That also skips the attempt to insert it in the SLB cache which is a nice side effect. Another way to fix the bug would be to make the entries in the SLB cache wider, so that we don't truncate the ESID. However this would be a more intrusive change as it alters the size and layout of the paca. This bug was fixed in upstream by commit f0f558b131db ("powerpc/mm: Preserve CFAR value on SLB miss caused by access to bogus address"), which changed the way we handle a bad EA entirely removing this bug in the process. Signed-off-by: Michael Ellerman <mpe@ellerman.id.au> Reviewed-by: Paul Mackerras <paulus@ozlabs.org> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
1 files changed, 10 insertions, 0 deletions
diff --git a/arch/powerpc/mm/slb_low.S b/arch/powerpc/mm/slb_low.S
index 4c48b487698c..0b48ce40d351 100644
--- a/arch/powerpc/mm/slb_low.S
+++ b/arch/powerpc/mm/slb_low.S
b slb_finish_load
8: /* invalid EA */
+ /*
+ * It's possible the bad EA is too large to fit in the SLB cache, which
+ * would mean we'd fail to invalidate it on context switch. So mark the
+ * SLB cache as full so we force a full flush. We also set cr7+eq to
+ * mark the address as a kernel address, so slb_finish_load() skips
+ * trying to insert it into the SLB cache.
+ */
+ crset 4*cr7+eq
li r10,0 /* BAD_VSID */
li r9,0 /* BAD_VSID */
li r11,SLB_VSID_USER /* flags don't much matter */