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Subject[tip: x86/urgent] x86/mm: Fix kern_addr_valid() to cope with existing but not present entries
The following commit has been merged into the x86/urgent branch of tip:

Commit-ID: 34b1999da935a33be6239226bfa6cd4f704c5c88
Gitweb: https://git.kernel.org/tip/34b1999da935a33be6239226bfa6cd4f704c5c88
Author: Mike Rapoport <rppt@linux.ibm.com>
AuthorDate: Thu, 19 Aug 2021 16:27:17 +03:00
Committer: Borislav Petkov <bp@suse.de>
CommitterDate: Wed, 08 Sep 2021 20:50:32 +02:00

x86/mm: Fix kern_addr_valid() to cope with existing but not present entries

Jiri Olsa reported a fault when running:

# cat /proc/kallsyms | grep ksys_read
ffffffff8136d580 T ksys_read
# objdump -d --start-address=0xffffffff8136d580 --stop-address=0xffffffff8136d590 /proc/kcore

/proc/kcore: file format elf64-x86-64

Segmentation fault

general protection fault, probably for non-canonical address 0xf887ffcbff000: 0000 [#1] SMP PTI
CPU: 12 PID: 1079 Comm: objdump Not tainted 5.14.0-rc5qemu+ #508
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.14.0-4.fc34 04/01/2014
RIP: 0010:kern_addr_valid
Call Trace:
read_kcore
? rcu_read_lock_sched_held
? rcu_read_lock_sched_held
? rcu_read_lock_sched_held
? trace_hardirqs_on
? rcu_read_lock_sched_held
? lock_acquire
? lock_acquire
? rcu_read_lock_sched_held
? lock_acquire
? rcu_read_lock_sched_held
? rcu_read_lock_sched_held
? rcu_read_lock_sched_held
? lock_release
? _raw_spin_unlock
? __handle_mm_fault
? rcu_read_lock_sched_held
? lock_acquire
? rcu_read_lock_sched_held
? lock_release
proc_reg_read
? vfs_read
vfs_read
ksys_read
do_syscall_64
entry_SYSCALL_64_after_hwframe

The fault happens because kern_addr_valid() dereferences existent but not
present PMD in the high kernel mappings.

Such PMDs are created when free_kernel_image_pages() frees regions larger
than 2Mb. In this case, a part of the freed memory is mapped with PMDs and
the set_memory_np_noalias() -> ... -> __change_page_attr() sequence will
mark the PMD as not present rather than wipe it completely.

Have kern_addr_valid() check whether higher level page table entries are
present before trying to dereference them to fix this issue and to avoid
similar issues in the future.

Stable backporting note:
------------------------

Note that the stable marking is for all active stable branches because
there could be cases where pagetable entries exist but are not valid -
see 9a14aefc1d28 ("x86: cpa, fix lookup_address"), for example. So make
sure to be on the safe side here and use pXY_present() accessors rather
than pXY_none() which could #GP when accessing pages in the direct map.

Also see:

c40a56a7818c ("x86/mm/init: Remove freed kernel image areas from alias mapping")

for more info.

Reported-by: Jiri Olsa <jolsa@redhat.com>
Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: David Hildenbrand <david@redhat.com>
Acked-by: Dave Hansen <dave.hansen@intel.com>
Tested-by: Jiri Olsa <jolsa@redhat.com>
Cc: <stable@vger.kernel.org> # 4.4+
Link: https://lkml.kernel.org/r/20210819132717.19358-1-rppt@kernel.org
---
arch/x86/mm/init_64.c | 6 +++---
1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
index ddeaba9..879886c 100644
--- a/arch/x86/mm/init_64.c
+++ b/arch/x86/mm/init_64.c
@@ -1433,18 +1433,18 @@ int kern_addr_valid(unsigned long addr)
return 0;

p4d = p4d_offset(pgd, addr);
- if (p4d_none(*p4d))
+ if (!p4d_present(*p4d))
return 0;

pud = pud_offset(p4d, addr);
- if (pud_none(*pud))
+ if (!pud_present(*pud))
return 0;

if (pud_large(*pud))
return pfn_valid(pud_pfn(*pud));

pmd = pmd_offset(pud, addr);
- if (pmd_none(*pmd))
+ if (!pmd_present(*pmd))
return 0;

if (pmd_large(*pmd))
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