Patch "MIPS: Sync icache & dcache in set_pte_at" has been added to the 4.5-stable tree

This is a note to let you know that I've just added the patch titled

MIPS: Sync icache & dcache in set_pte_at

to the 4.5-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
mips-sync-icache-dcache-in-set_pte_at.patch
and it can be found in the queue-4.5 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Paul Burton <[email protected]>
Date: Tue, 1 Mar 2016 02:37:59 +0000
Subject: MIPS: Sync icache & dcache in set_pte_at

From: Paul Burton <[email protected]>

commit 37d22a0d798b5c938b277d32cfd86dc231381342 upstream.

It's possible for pages to become visible prior to update_mmu_cache running if a thread within the same address space preempts the current thread or runs simultaneously on another CPU. That is, the following scenario is possible:

CPU0 CPU1

write to page
flush_dcache_page
flush_icache_page
set_pte_at
map page
update_mmu_cache

If CPU1 maps the page in between CPU0's set_pte_at, which marks it valid & visible, and update_mmu_cache where the dcache flush occurs then CPU1s icache will fill from stale data (unless it fills from the dcache, in which case all is good, but most MIPS CPUs don't have this property). Commit 4d46a67a3eb8 ("MIPS: Fix race condition in lazy cache flushing.") attempted to fix that by performing the dcache flush in flush_icache_page such that it occurs before the set_pte_at call makes the page visible. However it has the problem that not all code that writes to pages exposed to userland call flush_icache_page. There are many callers of set_pte_at under mm/ and only 2 of them do call flush_icache_page. Thus the race window between a page becoming visible & being coherent between the icache & dcache remains open in some cases.

To illustrate some of the cases, a WARN was added to __update_cache with this patch applied that triggered in cases where a page about to be flushed from the dcache was not the last page provided to flush_icache_page. That is, backtraces were obtained for cases in which the race window is left open without this patch. The 2 standout examples follow.

When forking a process:

[   15.271842] [<80417630>] __update_cache+0xcc/0x188
[   15.277274] [<80530394>] copy_page_range+0x56c/0x6ac
[   15.282861] [<8042936c>] copy_process.part.54+0xd40/0x17ac
[   15.289028] [<80429f80>] do_fork+0xe4/0x420
[   15.293747] [<80413808>] handle_sys+0x128/0x14c

When exec'ing an ELF binary:

[   14.445964] [<80417630>] __update_cache+0xcc/0x188
[   14.451369] [<80538d88>] move_page_tables+0x414/0x498
[   14.457075] [<8055d848>] setup_arg_pages+0x220/0x318
[   14.462685] [<805b0f38>] load_elf_binary+0x530/0x12a0
[   14.468374] [<8055ec3c>] search_binary_handler+0xbc/0x214
[   14.474444] [<8055f6c0>] do_execveat_common+0x43c/0x67c
[   14.480324] [<8055f938>] do_execve+0x38/0x44
[   14.485137] [<80413808>] handle_sys+0x128/0x14c

These code paths write into a page, call flush_dcache_page then call set_pte_at without flush_icache_page inbetween. The end result is that the icache can become corrupted & userland processes may execute unexpected or invalid code, typically resulting in a reserved instruction exception, a trap or a segfault.

Fix this race condition fully by performing any cache maintenance required to keep the icache & dcache in sync in set_pte_at, before the page is made valid. This has the added bonus of ensuring the cache maintenance always happens in one location, rather than being duplicated in flush_icache_page & update_mmu_cache. It also matches the way other architectures solve the same problem (see arm, ia64 & powerpc).

Patchwork: https://patchwork.linux-mips.org/patch/12722/

---
 arch/mips/include/asm/cacheflush.h |    6 ------
 arch/mips/include/asm/pgtable.h    |   26 +++++++++++++++++++++-----
 arch/mips/mm/cache.c               |   19 +++----------------
 3 files changed, 24 insertions(+), 27 deletions(-)

@@ -51,7 +51,6 @@ extern void (*flush_cache_range)(struct
 	unsigned long start, unsigned long end);
 extern void (*flush_cache_page)(struct vm_area_struct *vma, unsigned long page, unsigned long pfn);
 extern void __flush_dcache_page(struct page *page);
-extern void __flush_icache_page(struct vm_area_struct *vma, struct page *page);
 
 #define ARCH_IMPLEMENTS_FLUSH_DCACHE_PAGE 1
 static inline void flush_dcache_page(struct page *page)

@@ -77,11 +76,6 @@ static inline void flush_anon_page(struc
 static inline void flush_icache_page(struct vm_area_struct *vma,
 	struct page *page)
 {
-	if (!cpu_has_ic_fills_f_dc && (vma->vm_flags & VM_EXEC) &&
-	    Page_dcache_dirty(page)) {
-		__flush_icache_page(vma, page);
-		ClearPageDcacheDirty(page);
-	}
 }
 
 extern void (*flush_icache_range)(unsigned long start, unsigned long end);

@@ -127,10 +127,14 @@ do {									\
 	}								\
 } while(0)
 
+static inline void set_pte_at(struct mm_struct *mm, unsigned long addr,
+			      pte_t *ptep, pte_t pteval);
+
 #if defined(CONFIG_PHYS_ADDR_T_64BIT) && defined(CONFIG_CPU_MIPS32)
 
 #define pte_none(pte)		(!(((pte).pte_high) & ~_PAGE_GLOBAL))
 #define pte_present(pte)	((pte).pte_low & _PAGE_PRESENT)
+#define pte_no_exec(pte)	((pte).pte_low & _PAGE_NO_EXEC)
 
 static inline void set_pte(pte_t *ptep, pte_t pte)
 {

@@ -148,7 +152,6 @@ static inline void set_pte(pte_t *ptep,
 			buddy->pte_high |= _PAGE_GLOBAL;
 	}
 }
-#define set_pte_at(mm, addr, ptep, pteval) set_pte(ptep, pteval)
 
 static inline void pte_clear(struct mm_struct *mm, unsigned long addr, pte_t *ptep)
 {

@@ -166,6 +169,7 @@ static inline void pte_clear(struct mm_s
 
 #define pte_none(pte)		(!(pte_val(pte) & ~_PAGE_GLOBAL))
 #define pte_present(pte)	(pte_val(pte) & _PAGE_PRESENT)
+#define pte_no_exec(pte)	(pte_val(pte) & _PAGE_NO_EXEC)
 
 /*
  * Certain architectures need to do special things when pte's

@@ -218,7 +222,6 @@ static inline void set_pte(pte_t *ptep,
 	}
 #endif
 }
-#define set_pte_at(mm, addr, ptep, pteval) set_pte(ptep, pteval)
 
 static inline void pte_clear(struct mm_struct *mm, unsigned long addr, pte_t *ptep)
 {

@@ -234,6 +237,22 @@ static inline void pte_clear(struct mm_s
 }
 #endif
 
+static inline void set_pte_at(struct mm_struct *mm, unsigned long addr,
+			      pte_t *ptep, pte_t pteval)
+{
+	extern void __update_cache(unsigned long address, pte_t pte);
+
+	if (!pte_present(pteval))
+		goto cache_sync_done;
+
+	if (pte_present(*ptep) && (pte_pfn(*ptep) == pte_pfn(pteval)))
+		goto cache_sync_done;
+
+	__update_cache(addr, pteval);
+cache_sync_done:
+	set_pte(ptep, pteval);
+}
+
 /*
  * (pmds are folded into puds so this doesn't get actually called,
  * but the define is needed for a generic inline function.)

@@ -430,15 +449,12 @@ static inline pte_t pte_modify(pte_t pte
 
 extern void __update_tlb(struct vm_area_struct *vma, unsigned long address,
 	pte_t pte);
-extern void __update_cache(struct vm_area_struct *vma, unsigned long address,
-	pte_t pte);
 
 static inline void update_mmu_cache(struct vm_area_struct *vma,
 	unsigned long address, pte_t *ptep)
 {
 	pte_t pte = *ptep;
 	__update_tlb(vma, address, pte);
-	__update_cache(vma, address, pte);
 }
 
 static inline void update_mmu_cache_pmd(struct vm_area_struct *vma,

@@ -119,30 +119,17 @@ void __flush_anon_page(struct page *page
 
 EXPORT_SYMBOL(__flush_anon_page);
 
-void __flush_icache_page(struct vm_area_struct *vma, struct page *page)
-{
-	unsigned long addr;
-
-	if (PageHighMem(page))
-		return;
-
-	addr = (unsigned long) page_address(page);
-	flush_data_cache_page(addr);
-}
-EXPORT_SYMBOL_GPL(__flush_icache_page);
-
-void __update_cache(struct vm_area_struct *vma, unsigned long address,
-	pte_t pte)
+void __update_cache(unsigned long address, pte_t pte)
 {
 	struct page *page;
 	unsigned long pfn, addr;
-	int exec = (vma->vm_flags & VM_EXEC) && !cpu_has_ic_fills_f_dc;
+	int exec = !pte_no_exec(pte) && !cpu_has_ic_fills_f_dc;
 
 	pfn = pte_pfn(pte);
 	if (unlikely(!pfn_valid(pfn)))
 		return;
 	page = pfn_to_page(pfn);
-	if (page_mapping(page) && Page_dcache_dirty(page)) {
+	if (Page_dcache_dirty(page)) {
 		if (PageHighMem(page))
 			addr = (unsigned long)kmap_atomic(page);
 		else

Patches currently in stable-queue which might be from [email protected] are

queue-4.5/mips-disable-preemption-during-prctl-pr_set_fp_mode.patch queue-4.5/mips-fix-sigreturn-via-vdso-on-micromips-kernel.patch queue-4.5/mips-handle-highmem-pages-in-__update_cache.patch queue-4.5/mips-fix-watchpoint-restoration.patch queue-4.5/mips-build-micromips-vdso-for-micromips-kernels.patch queue-4.5/mips-math-emu-fix-jalr-emulation-when-rd-0.patch queue-4.5/mips-sync-icache-dcache-in-set_pte_at.patch queue-4.5/mips-fix-msa-ld_-st_-asm-macros-to-use-ptr_addu.patch queue-4.5/mips-prevent-restoration-of-msa-context-in-non-msa-kernels.patch queue-4.5/mips-use-copy_s.fmt-rather-than-copy_u.fmt.patch