[kernel] CVE-2012-2372 mm: 32bit PAE pmd walk vs populate SMP race (rhbz 822821 822825)
Josh Boyer
jwboyer at fedoraproject.org
Thu May 24 11:47:28 UTC 2012
commit 440886d0908f9f900d0fcd4b77d0838e940df7d6
Author: Josh Boyer <jwboyer at redhat.com>
Date: Thu May 24 07:43:48 2012 -0400
CVE-2012-2372 mm: 32bit PAE pmd walk vs populate SMP race (rhbz 822821 822825)
kernel.spec | 11 +-
...d-walk-vs-pmd_populate-SMP-race-condition.patch | 272 ++++++++++++++++++++
2 files changed, 282 insertions(+), 1 deletions(-)
---
diff --git a/kernel.spec b/kernel.spec
index 42854aa..0a91e02 100644
--- a/kernel.spec
+++ b/kernel.spec
@@ -62,7 +62,7 @@ Summary: The Linux kernel
# For non-released -rc kernels, this will be appended after the rcX and
# gitX tags, so a 3 here would become part of release "0.rcX.gitX.3"
#
-%global baserelease 1
+%global baserelease 2
%global fedora_build %{baserelease}
# base_sublevel is the kernel version we're starting with and patching
@@ -753,6 +753,9 @@ Patch22000: weird-root-dentry-name-debug.patch
#selinux ptrace child permissions
Patch22001: selinux-apply-different-permission-to-ptrace-child.patch
+#rhbz 822825 822821 CVE-2012-2372
+Patch22021: mm-pmd_read_atomic-fix-32bit-PAE-pmd-walk-vs-pmd_populate-SMP-race-condition.patch
+
# END OF PATCH DEFINITIONS
%endif
@@ -1451,6 +1454,9 @@ ApplyPatch highbank-export-clock-functions.patch
#vgaarb patches. blame mjg59
ApplyPatch vgaarb-vga_default_device.patch
+#rhbz 822825 822821 CVE-2012-2372
+ApplyPatch mm-pmd_read_atomic-fix-32bit-PAE-pmd-walk-vs-pmd_populate-SMP-race-condition.patch
+
# END OF PATCH APPLICATIONS
%endif
@@ -2301,6 +2307,9 @@ fi
# ||----w |
# || ||
%changelog
+* Thu May 24 2012 Josh Boyer <jwboyer at redhat.com>
+- CVE-2012-2372 mm: 32bit PAE pmd walk vs populate SMP race (rhbz 822821 822825)
+
* Thu May 24 2012 Peter Robinson <pbrobinson at fedoraproject.org>
- Don't build Nokia ARM device support
diff --git a/mm-pmd_read_atomic-fix-32bit-PAE-pmd-walk-vs-pmd_populate-SMP-race-condition.patch b/mm-pmd_read_atomic-fix-32bit-PAE-pmd-walk-vs-pmd_populate-SMP-race-condition.patch
new file mode 100644
index 0000000..49ff98a
--- /dev/null
+++ b/mm-pmd_read_atomic-fix-32bit-PAE-pmd-walk-vs-pmd_populate-SMP-race-condition.patch
@@ -0,0 +1,272 @@
+Path: news.gmane.org!not-for-mail
+From: Andrea Arcangeli <aarcange at redhat.com>
+Newsgroups: gmane.linux.kernel.mm
+Subject: [PATCH] mm: pmd_read_atomic: fix 32bit PAE pmd walk vs pmd_populate SMP race condition
+Date: Thu, 24 May 2012 01:39:01 +0200
+Lines: 208
+Approved: news at gmane.org
+Message-ID: <1337816341-30743-1-git-send-email-aarcange at redhat.com>
+References: <20120518230028.GF32479 at redhat.com>
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+Cc: Andrew Morton <akpm at linux-foundation.org>, Mel Gorman <mgorman at suse.de>,
+ Hugh Dickins <hughd at google.com>, Larry Woodman <lwoodman at redhat.com>,
+ Petr Matousek <pmatouse at redhat.com>,
+ Ulrich Obergfell <uobergfe at redhat.com>, Rik van Riel <riel at redhat.com>
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+
+When holding the mmap_sem for reading, pmd_offset_map_lock should only
+run on a pmd_t that has been read atomically from the pmdp
+pointer, otherwise we may read only half of it leading to this crash.
+
+PID: 11679 TASK: f06e8000 CPU: 3 COMMAND: "do_race_2_panic"
+ #0 [f06a9dd8] crash_kexec at c049b5ec
+ #1 [f06a9e2c] oops_end at c083d1c2
+ #2 [f06a9e40] no_context at c0433ded
+ #3 [f06a9e64] bad_area_nosemaphore at c043401a
+ #4 [f06a9e6c] __do_page_fault at c0434493
+ #5 [f06a9eec] do_page_fault at c083eb45
+ #6 [f06a9f04] error_code (via page_fault) at c083c5d5
+ EAX: 01fb470c EBX: fff35000 ECX: 00000003 EDX: 00000100 EBP:
+ 00000000
+ DS: 007b ESI: 9e201000 ES: 007b EDI: 01fb4700 GS: 00e0
+ CS: 0060 EIP: c083bc14 ERR: ffffffff EFLAGS: 00010246
+ #7 [f06a9f38] _spin_lock at c083bc14
+ #8 [f06a9f44] sys_mincore at c0507b7d
+ #9 [f06a9fb0] system_call at c083becd
+ start len
+ EAX: ffffffda EBX: 9e200000 ECX: 00001000 EDX: 6228537f
+ DS: 007b ESI: 00000000 ES: 007b EDI: 003d0f00
+ SS: 007b ESP: 62285354 EBP: 62285388 GS: 0033
+ CS: 0073 EIP: 00291416 ERR: 000000da EFLAGS: 00000286
+
+This should be a longstanding bug affecting x86 32bit PAE without
+THP. Only archs with 64bit large pmd_t and 32bit unsigned long should
+be affected.
+
+With THP enabled the barrier() in
+pmd_none_or_trans_huge_or_clear_bad() would partly hide the bug when
+the pmd transition from none to stable, by forcing a re-read of the
+*pmd in pmd_offset_map_lock, but when THP is enabled a new set of
+problem arises by the fact could then transition freely in any of the
+none, pmd_trans_huge or pmd_trans_stable states. So making the barrier
+in pmd_none_or_trans_huge_or_clear_bad() unconditional isn't good idea
+and it would be a flakey solution.
+
+This should be fully fixed by introducing a pmd_read_atomic that reads
+the pmd in order with THP disabled, or by reading the pmd atomically
+with cmpxchg8b with THP enabled.
+
+Luckily this new race condition only triggers in the places that must
+already be covered by pmd_none_or_trans_huge_or_clear_bad() so the fix
+is localized there but this bug is not related to THP.
+
+NOTE: this can trigger on x86 32bit systems with PAE enabled with more
+than 4G of ram, otherwise the high part of the pmd will never risk to
+be truncated because it would be zero at all times, in turn so hiding
+the SMP race.
+
+This bug was discovered and fully debugged by Ulrich, quote:
+
+----
+[..]
+pmd_none_or_trans_huge_or_clear_bad() loads the content of edx and
+eax.
+
+ 496 static inline int pmd_none_or_trans_huge_or_clear_bad(pmd_t
+ *pmd)
+ 497 {
+ 498 /* depend on compiler for an atomic pmd read */
+ 499 pmd_t pmdval = *pmd;
+
+ // edi = pmd pointer
+0xc0507a74 <sys_mincore+548>: mov 0x8(%esp),%edi
+...
+ // edx = PTE page table high address
+0xc0507a84 <sys_mincore+564>: mov 0x4(%edi),%edx
+...
+ // eax = PTE page table low address
+0xc0507a8e <sys_mincore+574>: mov (%edi),%eax
+
+[..]
+
+Please note that the PMD is not read atomically. These are two "mov"
+instructions where the high order bits of the PMD entry are fetched
+first. Hence, the above machine code is prone to the following race.
+
+- The PMD entry {high|low} is 0x0000000000000000.
+ The "mov" at 0xc0507a84 loads 0x00000000 into edx.
+
+- A page fault (on another CPU) sneaks in between the two "mov"
+ instructions and instantiates the PMD.
+
+- The PMD entry {high|low} is now 0x00000003fda38067.
+ The "mov" at 0xc0507a8e loads 0xfda38067 into eax.
+----
+
+Reported-by: Ulrich Obergfell <uobergfe at redhat.com>
+Signed-off-by: Andrea Arcangeli <aarcange at redhat.com>
+---
+ arch/x86/include/asm/pgtable-3level.h | 50 +++++++++++++++++++++++++++++++++
+ include/asm-generic/pgtable.h | 22 +++++++++++++-
+ 2 files changed, 70 insertions(+), 2 deletions(-)
+
+diff --git a/arch/x86/include/asm/pgtable-3level.h b/arch/x86/include/asm/pgtable-3level.h
+index effff47..43876f1 100644
+--- a/arch/x86/include/asm/pgtable-3level.h
++++ b/arch/x86/include/asm/pgtable-3level.h
+@@ -31,6 +31,56 @@ static inline void native_set_pte(pte_t *ptep, pte_t pte)
+ ptep->pte_low = pte.pte_low;
+ }
+
++#define pmd_read_atomic pmd_read_atomic
++/*
++ * pte_offset_map_lock on 32bit PAE kernels was reading the pmd_t with
++ * a "*pmdp" dereference done by gcc. Problem is, in certain places
++ * where pte_offset_map_lock is called, concurrent page faults are
++ * allowed, if the mmap_sem is hold for reading. An example is mincore
++ * vs page faults vs MADV_DONTNEED. On the page fault side
++ * pmd_populate rightfully does a set_64bit, but if we're reading the
++ * pmd_t with a "*pmdp" on the mincore side, a SMP race can happen
++ * because gcc will not read the 64bit of the pmd atomically. To fix
++ * this all places running pmd_offset_map_lock() while holding the
++ * mmap_sem in read mode, shall read the pmdp pointer using this
++ * function to know if the pmd is null nor not, and in turn to know if
++ * they can run pmd_offset_map_lock or pmd_trans_huge or other pmd
++ * operations.
++ *
++ * Without THP if the mmap_sem is hold for reading, the
++ * pmd can only transition from null to not null while pmd_read_atomic runs.
++ * So there's no need of literally reading it atomically.
++ *
++ * With THP if the mmap_sem is hold for reading, the pmd can become
++ * THP or null or point to a pte (and in turn become "stable") at any
++ * time under pmd_read_atomic, so it's mandatory to read it atomically
++ * with cmpxchg8b.
++ */
++#ifndef CONFIG_TRANSPARENT_HUGEPAGE
++static inline pmd_t pmd_read_atomic(pmd_t *pmdp)
++{
++ pmdval_t ret;
++ u32 *tmp = (u32 *)pmdp;
++
++ ret = (pmdval_t) (*tmp);
++ if (ret) {
++ /*
++ * If the low part is null, we must not read the high part
++ * or we can end up with a partial pmd.
++ */
++ smp_rmb();
++ ret |= ((pmdval_t)*(tmp + 1)) << 32;
++ }
++
++ return (pmd_t) { ret };
++}
++#else /* CONFIG_TRANSPARENT_HUGEPAGE */
++static inline pmd_t pmd_read_atomic(pmd_t *pmdp)
++{
++ return (pmd_t) { atomic64_read((atomic64_t *)pmdp) };
++}
++#endif /* CONFIG_TRANSPARENT_HUGEPAGE */
++
+ static inline void native_set_pte_atomic(pte_t *ptep, pte_t pte)
+ {
+ set_64bit((unsigned long long *)(ptep), native_pte_val(pte));
+diff --git a/include/asm-generic/pgtable.h b/include/asm-generic/pgtable.h
+index 125c54e..fa596d9 100644
+--- a/include/asm-generic/pgtable.h
++++ b/include/asm-generic/pgtable.h
+@@ -446,6 +446,18 @@ static inline int pmd_write(pmd_t pmd)
+ #endif /* __HAVE_ARCH_PMD_WRITE */
+ #endif /* CONFIG_TRANSPARENT_HUGEPAGE */
+
++#ifndef pmd_read_atomic
++static inline pmd_t pmd_read_atomic(pmd_t *pmdp)
++{
++ /*
++ * Depend on compiler for an atomic pmd read. NOTE: this is
++ * only going to work, if the pmdval_t isn't larger than
++ * an unsigned long.
++ */
++ return *pmdp;
++}
++#endif
++
+ /*
+ * This function is meant to be used by sites walking pagetables with
+ * the mmap_sem hold in read mode to protect against MADV_DONTNEED and
+@@ -459,11 +471,17 @@ static inline int pmd_write(pmd_t pmd)
+ * undefined so behaving like if the pmd was none is safe (because it
+ * can return none anyway). The compiler level barrier() is critically
+ * important to compute the two checks atomically on the same pmdval.
++ *
++ * For 32bit kernels with a 64bit large pmd_t this automatically takes
++ * care of reading the pmd atomically to avoid SMP race conditions
++ * against pmd_populate() when the mmap_sem is hold for reading by the
++ * caller (a special atomic read not done by "gcc" as in the generic
++ * version above, is also needed when THP is disabled because the page
++ * fault can populate the pmd from under us).
+ */
+ static inline int pmd_none_or_trans_huge_or_clear_bad(pmd_t *pmd)
+ {
+- /* depend on compiler for an atomic pmd read */
+- pmd_t pmdval = *pmd;
++ pmd_t pmdval = pmd_read_atomic(pmd);
+ /*
+ * The barrier will stabilize the pmdval in a register or on
+ * the stack so that it will stop changing under the code.
+
+--
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