From f077aaf0754bcba0fffdbd925bc12f09cd1e38aa Mon Sep 17 00:00:00 2001 From: Paul Mackerras Date: Fri, 2 Sep 2016 21:47:59 +1000 Subject: [PATCH] powerpc/mm: Don't alias user region to other regions below PAGE_OFFSET In commit c60ac5693c47 ("powerpc: Update kernel VSID range", 2013-03-13) we lost a check on the region number (the top four bits of the effective address) for addresses below PAGE_OFFSET. That commit replaced a check that the top 18 bits were all zero with a check that bits 46 - 59 were zero (performed for all addresses, not just user addresses). This means that userspace can access an address like 0x1000_0xxx_xxxx_xxxx and we will insert a valid SLB entry for it. The VSID used will be the same as if the top 4 bits were 0, but the page size will be some random value obtained by indexing beyond the end of the mm_ctx_high_slices_psize array in the paca. If that page size is the same as would be used for region 0, then userspace just has an alias of the region 0 space. If the page size is different, then no HPTE will be found for the access, and the process will get a SIGSEGV (since hash_page_mm() will refuse to create a HPTE for the bogus address). The access beyond the end of the mm_ctx_high_slices_psize can be at most 5.5MB past the array, and so will be in RAM somewhere. Since the access is a load performed in real mode, it won't fault or crash the kernel. At most this bug could perhaps leak a little bit of information about blocks of 32 bytes of memory located at offsets of i * 512kB past the paca->mm_ctx_high_slices_psize array, for 1 <= i <= 11. Fixes: c60ac5693c47 ("powerpc: Update kernel VSID range") Cc: stable@vger.kernel.org # v3.9+ Signed-off-by: Paul Mackerras Reviewed-by: Aneesh Kumar K.V Signed-off-by: Michael Ellerman --- arch/powerpc/mm/slb_low.S | 7 ++++++- 1 file changed, 6 insertions(+), 1 deletion(-) diff --git a/arch/powerpc/mm/slb_low.S b/arch/powerpc/mm/slb_low.S index dfdb90cb4403..9f1983404e1a 100644 --- a/arch/powerpc/mm/slb_low.S +++ b/arch/powerpc/mm/slb_low.S @@ -113,7 +113,12 @@ BEGIN_FTR_SECTION END_MMU_FTR_SECTION_IFCLR(MMU_FTR_1T_SEGMENT) b slb_finish_load_1T -0: +0: /* + * For userspace addresses, make sure this is region 0. + */ + cmpdi r9, 0 + bne 8f + /* when using slices, we extract the psize off the slice bitmaps * and then we need to get the sllp encoding off the mmu_psize_defs * array. -- 2.20.1