path: root/arch
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authorMichael Ellerman <>2021-08-15 14:10:24 +1000
committerMichael Ellerman <>2021-08-19 09:41:54 +1000
commit9f7853d7609d59172eecfc5e7ccf503bc1b690bd (patch)
tree8fb9ea8872a5bae62fd937775bab16adb3e4fe18 /arch
parentef486bf448a057a6e2d50e40ae879f7add6585da (diff)
powerpc/mm: Fix set_memory_*() against concurrent accesses
Laurent reported that STRICT_MODULE_RWX was causing intermittent crashes on one of his systems: kernel tried to execute exec-protected page (c008000004073278) - exploit attempt? (uid: 0) BUG: Unable to handle kernel instruction fetch Faulting instruction address: 0xc008000004073278 Oops: Kernel access of bad area, sig: 11 [#1] LE PAGE_SIZE=64K MMU=Radix SMP NR_CPUS=2048 NUMA pSeries Modules linked in: drm virtio_console fuse drm_panel_orientation_quirks ... CPU: 3 PID: 44 Comm: kworker/3:1 Not tainted 5.14.0-rc4+ #12 Workqueue: events control_work_handler [virtio_console] NIP: c008000004073278 LR: c008000004073278 CTR: c0000000001e9de0 REGS: c00000002e4ef7e0 TRAP: 0400 Not tainted (5.14.0-rc4+) MSR: 800000004280b033 <SF,VEC,VSX,EE,FP,ME,IR,DR,RI,LE> CR: 24002822 XER: 200400cf ... NIP fill_queue+0xf0/0x210 [virtio_console] LR fill_queue+0xf0/0x210 [virtio_console] Call Trace: fill_queue+0xb4/0x210 [virtio_console] (unreliable) add_port+0x1a8/0x470 [virtio_console] control_work_handler+0xbc/0x1e8 [virtio_console] process_one_work+0x290/0x590 worker_thread+0x88/0x620 kthread+0x194/0x1a0 ret_from_kernel_thread+0x5c/0x64 Jordan, Fabiano & Murilo were able to reproduce and identify that the problem is caused by the call to module_enable_ro() in do_init_module(), which happens after the module's init function has already been called. Our current implementation of change_page_attr() is not safe against concurrent accesses, because it invalidates the PTE before flushing the TLB and then installing the new PTE. That leaves a window in time where there is no valid PTE for the page, if another CPU tries to access the page at that time we see something like the fault above. We can't simply switch to set_pte_at()/flush TLB, because our hash MMU code doesn't handle a set_pte_at() of a valid PTE. See [1]. But we do have pte_update(), which replaces the old PTE with the new, meaning there's no window where the PTE is invalid. And the hash MMU version hash__pte_update() deals with synchronising the hash page table correctly. [1]: Fixes: 1f9ad21c3b38 ("powerpc/mm: Implement set_memory() routines") Reported-by: Laurent Vivier <> Reviewed-by: Christophe Leroy <> Reviewed-by: Murilo Opsfelder Ara├║jo <> Tested-by: Laurent Vivier <> Signed-off-by: Fabiano Rosas <> Signed-off-by: Michael Ellerman <> Link:
Diffstat (limited to 'arch')
1 files changed, 10 insertions, 13 deletions
diff --git a/arch/powerpc/mm/pageattr.c b/arch/powerpc/mm/pageattr.c
index 0876216ceee6..edea388e9d3f 100644
--- a/arch/powerpc/mm/pageattr.c
+++ b/arch/powerpc/mm/pageattr.c
@@ -18,16 +18,12 @@
* Updates the attributes of a page in three steps:
- * 1. invalidate the page table entry
- * 2. flush the TLB
- * 3. install the new entry with the updated attributes
- *
- * Invalidating the pte means there are situations where this will not work
- * when in theory it should.
- * For example:
- * - removing write from page whilst it is being executed
- * - setting a page read-only whilst it is being read by another CPU
+ * 1. take the page_table_lock
+ * 2. install the new entry with the updated attributes
+ * 3. flush the TLB
+ * This sequence is safe against concurrent updates, and also allows updating the
+ * attributes of a page currently being executed or accessed.
static int change_page_attr(pte_t *ptep, unsigned long addr, void *data)
@@ -36,9 +32,7 @@ static int change_page_attr(pte_t *ptep, unsigned long addr, void *data)
- /* invalidate the PTE so it's safe to modify */
- pte = ptep_get_and_clear(&init_mm, addr, ptep);
- flush_tlb_kernel_range(addr, addr + PAGE_SIZE);
+ pte = ptep_get(ptep);
/* modify the PTE bits as desired, then apply */
switch (action) {
@@ -59,11 +53,14 @@ static int change_page_attr(pte_t *ptep, unsigned long addr, void *data)
- set_pte_at(&init_mm, addr, ptep, pte);
+ pte_update(&init_mm, addr, ptep, ~0UL, pte_val(pte), 0);
/* See ptesync comment in radix__set_pte_at() */
if (radix_enabled())
asm volatile("ptesync": : :"memory");
+ flush_tlb_kernel_range(addr, addr + PAGE_SIZE);
return 0;