[PATCH v2 3/5] x86/alternative: exclude text poking against change_page_attr()

From: Pedro Falcato <[email protected]>

From time to time, the following BUG can be observed[0]:

> kernel BUG at arch/x86/kernel/alternative.c:2576!
> Oops: invalid opcode: 0000 [#1] SMP NOPTI
> CPU: 0 UID: 0 PID: 355 Comm: (udev-worker) Not tainted 7.1.3-1-default #1 PREEMPT(full) openSUSE Tumbleweed  8c1795b03ec64f997e57a8ad38b1161e3b98da64
> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS unknown 02/02/2022
> RIP: 0010:__text_poke+0x2aa/0x450
> Call Trace:
>  <TASK>
>  smp_text_poke_batch_finish+0x2a7/0x320
>  __static_call_transform+0xb7/0x220
>  arch_static_call_transform+0x5b/0xb0
>  __static_call_init+0xe9/0x270
>  static_call_module_notify+0x11f/0x150
>  notifier_call_chain+0x61/0xe0
>  blocking_notifier_call_chain_robust+0x63/0xc0
>  load_module+0x1c92/0x20c0
>  init_module_from_file+0xd8/0x140
>  idempotent_init_module+0x100/0x2f0
>  __x64_sys_finit_module+0x71/0xe0
>  do_syscall_64+0xe1/0x610
>  entry_SYSCALL_64_after_hwframe+0x76/0x7e

which matches the following BUG_ON in alternative.c:
	/*
	 * If something went wrong, crash and burn since recovery paths are not
	 * implemented.
	 */
	BUG_ON(!pages[0] || (cross_page_boundary && !pages[1]));

This can happen if vmalloc_to_page() fails, for any reason. Such can happen
if text poking races with CPA, which can possibly result in the collapsing
of page tables (or breaking of PMD hugepages). It is not a problem for most
users of vmalloc_to_page() (they solely own the vmalloc'd range) but, when
CONFIG_ARCH_HAS_EXECMEM_ROX=y, various modules own a single execmem vmalloc
range, and can call set_memory_*() in parallel on it. This can happen to
race against __text_poke and cause havoc in vmalloc_to_page().

Fix it by excluding against CPA using the init_mm mmap read lock.

Fixes: 64f6a4e10c05 ("x86: re-enable EXECMEM_ROX support")
Reported-by: Jiri Slaby <[email protected]>
Link: https://bugzilla.opensuse.org/show_bug.cgi?id=1271202 [0]
Reported-by: Steffen Dirkwinkel <[email protected]>
Link: https://lore.kernel.org/linux-mm/[email protected]/
Cc: [email protected]
Co-developed-by: "Lorenzo Stoakes (ARM)" <[email protected]>
Signed-off-by: "Lorenzo Stoakes (ARM)" <[email protected]>
Signed-off-by: Pedro Falcato <[email protected]>
Signed-off-by: Mike Rapoport (Microsoft) <[email protected]>
---
 arch/x86/kernel/alternative.c | 39 ++++++++++++++++++++++++++++++++++++---
 1 file changed, 36 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index 62936a3bde19..f81d6bc90a6f 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -6,6 +6,9 @@
 #include <linux/vmalloc.h>
 #include <linux/memory.h>
 #include <linux/execmem.h>
+#include <linux/cleanup.h>
+#include <linux/kgdb.h>
+#include <linux/mmap_lock.h>
 
 #include <asm/text-patching.h>
 #include <asm/insn.h>
@@ -2543,6 +2546,38 @@ static void text_poke_memset(void *dst, const void *src, size_t len)
 
 typedef void text_poke_f(void *dst, const void *src, size_t len);
 
+static void __poke_vmalloc_pages(struct page **pages, void *addr,
+				 bool cross_page_boundary)
+{
+	pages[0] = vmalloc_to_page(addr);
+	if (cross_page_boundary)
+		pages[1] = vmalloc_to_page(addr + PAGE_SIZE);
+}
+
+static void poke_vmalloc_pages(struct page **pages, void *addr,
+			       bool cross_page_boundary)
+{
+	if (in_dbg_master()) {
+		/*
+		 * If called from kgdb cannot sleep, but all other CPUs stopped
+		 * anyway so safe to proceed without locks
+		 */
+		__poke_vmalloc_pages(pages, addr, cross_page_boundary);
+	} else {
+		/*
+		 * execmem ROX ranges are shared between modules and can be
+		 * collapsed to huge PMD entries, and this collapse can happen
+		 * concurrently with a racing set_memory_rox().
+		 *
+		 * Prevent vmalloc_to_page() from racing by acquiring an
+		 * init_mm read lock which pairs with the init_mm write lock in
+		 * cpa_collapse_large_pages().
+		 */
+		guard(mmap_read_lock)(&init_mm);
+		__poke_vmalloc_pages(pages, addr, cross_page_boundary);
+	}
+}
+
 static void *__text_poke(text_poke_f func, void *addr, const void *src, size_t len)
 {
 	bool cross_page_boundary = offset_in_page(addr) + len > PAGE_SIZE;
@@ -2560,9 +2595,7 @@ static void *__text_poke(text_poke_f func, void *addr, const void *src, size_t l
 	BUG_ON(!after_bootmem);
 
 	if (!core_kernel_text((unsigned long)addr)) {
-		pages[0] = vmalloc_to_page(addr);
-		if (cross_page_boundary)
-			pages[1] = vmalloc_to_page(addr + PAGE_SIZE);
+		poke_vmalloc_pages(pages, addr, cross_page_boundary);
 	} else {
 		pages[0] = virt_to_page(addr);
 		WARN_ON(!PageReserved(pages[0]));

-- 
2.53.0