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authorPavel Tatashin <pasha.tatashin@soleen.com>2019-11-19 17:10:06 -0500
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>2019-11-24 08:17:00 +0100
commita291d917030f4e41fc39637f7e8e21234bd22716 (patch)
tree20551fd7407fb72cd59ce47f3a6cdf7842aa38e1
parent240bff9e72e9057fc2bda40d1a651fd2ac5b67bb (diff)
arm64: uaccess: Ensure PAN is re-enabled after unhandled uaccess fault
commit 94bb804e1e6f0a9a77acf20d7c70ea141c6c821e upstream. A number of our uaccess routines ('__arch_clear_user()' and '__arch_copy_{in,from,to}_user()') fail to re-enable PAN if they encounter an unhandled fault whilst accessing userspace. For CPUs implementing both hardware PAN and UAO, this bug has no effect when both extensions are in use by the kernel. For CPUs implementing hardware PAN but not UAO, this means that a kernel using hardware PAN may execute portions of code with PAN inadvertently disabled, opening us up to potential security vulnerabilities that rely on userspace access from within the kernel which would usually be prevented by this mechanism. In other words, parts of the kernel run the same way as they would on a CPU without PAN implemented/emulated at all. For CPUs not implementing hardware PAN and instead relying on software emulation via 'CONFIG_ARM64_SW_TTBR0_PAN=y', the impact is unfortunately much worse. Calling 'schedule()' with software PAN disabled means that the next task will execute in the kernel using the page-table and ASID of the previous process even after 'switch_mm()', since the actual hardware switch is deferred until return to userspace. At this point, or if there is a intermediate call to 'uaccess_enable()', the page-table and ASID of the new process are installed. Sadly, due to the changes introduced by KPTI, this is not an atomic operation and there is a very small window (two instructions) where the CPU is configured with the page-table of the old task and the ASID of the new task; a speculative access in this state is disastrous because it would corrupt the TLB entries for the new task with mappings from the previous address space. As Pavel explains: | I was able to reproduce memory corruption problem on Broadcom's SoC | ARMv8-A like this: | | Enable software perf-events with PERF_SAMPLE_CALLCHAIN so userland's | stack is accessed and copied. | | The test program performed the following on every CPU and forking | many processes: | | unsigned long *map = mmap(NULL, PAGE_SIZE, PROT_READ|PROT_WRITE, | MAP_SHARED | MAP_ANONYMOUS, -1, 0); | map[0] = getpid(); | sched_yield(); | if (map[0] != getpid()) { | fprintf(stderr, "Corruption detected!"); | } | munmap(map, PAGE_SIZE); | | From time to time I was getting map[0] to contain pid for a | different process. Ensure that PAN is re-enabled when returning after an unhandled user fault from our uaccess routines. Cc: Catalin Marinas <catalin.marinas@arm.com> Reviewed-by: Mark Rutland <mark.rutland@arm.com> Tested-by: Mark Rutland <mark.rutland@arm.com> Cc: <stable@vger.kernel.org> Fixes: 338d4f49d6f7 ("arm64: kernel: Add support for Privileged Access Never") Signed-off-by: Pavel Tatashin <pasha.tatashin@soleen.com> [will: rewrote commit message] Signed-off-by: Will Deacon <will@kernel.org> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
-rw-r--r--arch/arm64/lib/clear_user.S1
-rw-r--r--arch/arm64/lib/copy_from_user.S1
-rw-r--r--arch/arm64/lib/copy_in_user.S1
-rw-r--r--arch/arm64/lib/copy_to_user.S1
4 files changed, 4 insertions, 0 deletions
diff --git a/arch/arm64/lib/clear_user.S b/arch/arm64/lib/clear_user.S
index 10415572e82f..322b55664cca 100644
--- a/arch/arm64/lib/clear_user.S
+++ b/arch/arm64/lib/clear_user.S
@@ -48,5 +48,6 @@ EXPORT_SYMBOL(__arch_clear_user)
.section .fixup,"ax"
.align 2
9: mov x0, x2 // return the original size
+ uaccess_disable_not_uao x2, x3
ret
.previous
diff --git a/arch/arm64/lib/copy_from_user.S b/arch/arm64/lib/copy_from_user.S
index 680e74409ff9..8472dc7798b3 100644
--- a/arch/arm64/lib/copy_from_user.S
+++ b/arch/arm64/lib/copy_from_user.S
@@ -66,5 +66,6 @@ EXPORT_SYMBOL(__arch_copy_from_user)
.section .fixup,"ax"
.align 2
9998: sub x0, end, dst // bytes not copied
+ uaccess_disable_not_uao x3, x4
ret
.previous
diff --git a/arch/arm64/lib/copy_in_user.S b/arch/arm64/lib/copy_in_user.S
index 0bedae3f3792..8e0355c1e318 100644
--- a/arch/arm64/lib/copy_in_user.S
+++ b/arch/arm64/lib/copy_in_user.S
@@ -68,5 +68,6 @@ EXPORT_SYMBOL(__arch_copy_in_user)
.section .fixup,"ax"
.align 2
9998: sub x0, end, dst // bytes not copied
+ uaccess_disable_not_uao x3, x4
ret
.previous
diff --git a/arch/arm64/lib/copy_to_user.S b/arch/arm64/lib/copy_to_user.S
index 2d88c736e8f2..6085214654dc 100644
--- a/arch/arm64/lib/copy_to_user.S
+++ b/arch/arm64/lib/copy_to_user.S
@@ -65,5 +65,6 @@ EXPORT_SYMBOL(__arch_copy_to_user)
.section .fixup,"ax"
.align 2
9998: sub x0, end, dst // bytes not copied
+ uaccess_disable_not_uao x3, x4
ret
.previous