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[PATCH 5.0 130/137] powerpc/book3s/64: check for NULL pointer in pgd_alloc()

From: Rick Lindsley <ricklind@xxxxxxxxxxxxxxxxxx>

commit f39356261c265a0689d7ee568132d516e8b6cecc upstream.

When the memset code was added to pgd_alloc(), it failed to consider
that kmem_cache_alloc() can return NULL. It's uncommon, but not
impossible under heavy memory contention. Example oops:

  Unable to handle kernel paging request for data at address 0x00000000
  Faulting instruction address: 0xc0000000000a4000
  Oops: Kernel access of bad area, sig: 11 [#1]
  LE SMP NR_CPUS=2048 NUMA pSeries
  CPU: 70 PID: 48471 Comm: entrypoint.sh Kdump: loaded Not tainted 4.14.0-115.6.1.el7a.ppc64le #1
  task: c000000334a00000 task.stack: c000000331c00000
  NIP:  c0000000000a4000 LR: c00000000012f43c CTR: 0000000000000020
  REGS: c000000331c039c0 TRAP: 0300   Not tainted  (4.14.0-115.6.1.el7a.ppc64le)
  MSR:  800000010280b033 <SF,VEC,VSX,EE,FP,ME,IR,DR,RI,LE,TM[E]>  CR: 44022840  XER: 20040000
  CFAR: c000000000008874 DAR: 0000000000000000 DSISR: 42000000 SOFTE: 1
  NIP [c0000000000a4000] memset+0x68/0x104
  LR [c00000000012f43c] mm_init+0x27c/0x2f0
  Call Trace:
    mm_init+0x260/0x2f0 (unreliable)
  Instruction dump:
  409e000c b0860000 38c60002 409d000c 90860000 38c60004 78a0d183 78a506a0
  7c0903a6 41820034 60000000 60420000 <f8860000> f8860008 f8860010 f8860018

Fixes: fc5c2f4a55a2 ("powerpc/mm/hash64: Zero PGD pages on allocation")
Cc: stable@xxxxxxxxxxxxxxx # v4.16+
Signed-off-by: Rick Lindsley <ricklind@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

 arch/powerpc/include/asm/book3s/64/pgalloc.h |    3 +++
 1 file changed, 3 insertions(+)

--- a/arch/powerpc/include/asm/book3s/64/pgalloc.h
+++ b/arch/powerpc/include/asm/book3s/64/pgalloc.h
@@ -81,6 +81,9 @@ static inline pgd_t *pgd_alloc(struct mm
 	pgd = kmem_cache_alloc(PGT_CACHE(PGD_INDEX_SIZE),
 			       pgtable_gfp_flags(mm, GFP_KERNEL));
+	if (unlikely(!pgd))
+		return pgd;
 	 * Don't scan the PGD for pointers, it contains references to PUDs but
 	 * those references are not full pointers and so can't be recognised by