[PATCH] x86/boot/64: clang: use fixup_pointer() to access next_early_pgt

[PATCH] x86/boot/64: clang: use fixup_pointer() to access next_early_pgt

[Alexander Potapenko]

__startup_64() is normally using fixup_pointer() to access globals in a
position-independent fashion. However |next_early_pgt| was accessed
directly, which wasn't guaranteed to work.

Luckily GCC was generating a R_X86_64_PC32 PC-relative relocation for
|next_early_pgt|, but Clang emitted a R_X86_64_32S, which led to
accessing invalid memory and rebooting the kernel.

Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Michael Davidson <md@google.com>
Fixes: c88d71508e36 ("x86/boot/64: Rewrite startup_64() in C")
Signed-off-by: Alexander Potapenko <glider@google.com>
---
 arch/x86/kernel/head64.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kernel/head64.c b/arch/x86/kernel/head64.c
index 46c3c73e7f43..9ba79543d9ee 100644
--- a/arch/x86/kernel/head64.c
+++ b/arch/x86/kernel/head64.c
@@ -53,6 +53,7 @@ void __head __startup_64(unsigned long physaddr)
 	pudval_t *pud;
 	pmdval_t *pmd, pmd_entry;
 	int i;
+	unsigned int *next_pgt_ptr;
 
 	/* Is the address too large? */
 	if (physaddr >> MAX_PHYSMEM_BITS)
@@ -91,9 +92,9 @@ void __head __startup_64(unsigned long physaddr)
 	 * creates a bunch of nonsense entries but that is fine --
 	 * it avoids problems around wraparound.
 	 */
-
-	pud = fixup_pointer(early_dynamic_pgts[next_early_pgt++], physaddr);
-	pmd = fixup_pointer(early_dynamic_pgts[next_early_pgt++], physaddr);
+	next_pgt_ptr = fixup_pointer(&next_early_pgt, physaddr);
+	pud = fixup_pointer(early_dynamic_pgts[(*next_pgt_ptr)++], physaddr);
+	pmd = fixup_pointer(early_dynamic_pgts[(*next_pgt_ptr)++], physaddr);
 
 	if (IS_ENABLED(CONFIG_X86_5LEVEL)) {
 		p4d = fixup_pointer(early_dynamic_pgts[next_early_pgt++], physaddr);
-- 
2.14.1.480.gb18f417b89-goog

Re: [PATCH] x86/boot/64: clang: use fixup_pointer() to access next_early_pgt

[Alexander Potapenko]

On Wed, Aug 16, 2017 at 9:08 PM, Alexander Potapenko <glider@google.com> wrote:
> __startup_64() is normally using fixup_pointer() to access globals in a
> position-independent fashion. However |next_early_pgt| was accessed
> directly, which wasn't guaranteed to work.
>
> Luckily GCC was generating a R_X86_64_PC32 PC-relative relocation for
> |next_early_pgt|, but Clang emitted a R_X86_64_32S, which led to
> accessing invalid memory and rebooting the kernel.
>
> Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> Cc: Dmitry Vyukov <dvyukov@google.com>
> Cc: Michael Davidson <md@google.com>
> Fixes: c88d71508e36 ("x86/boot/64: Rewrite startup_64() in C")
> Signed-off-by: Alexander Potapenko <glider@google.com>
> ---
>  arch/x86/kernel/head64.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/kernel/head64.c b/arch/x86/kernel/head64.c
> index 46c3c73e7f43..9ba79543d9ee 100644
> --- a/arch/x86/kernel/head64.c
> +++ b/arch/x86/kernel/head64.c
> @@ -53,6 +53,7 @@ void __head __startup_64(unsigned long physaddr)
>         pudval_t *pud;
>         pmdval_t *pmd, pmd_entry;
>         int i;
> +       unsigned int *next_pgt_ptr;
>
>         /* Is the address too large? */
>         if (physaddr >> MAX_PHYSMEM_BITS)
> @@ -91,9 +92,9 @@ void __head __startup_64(unsigned long physaddr)
>          * creates a bunch of nonsense entries but that is fine --
>          * it avoids problems around wraparound.
>          */
> -
> -       pud = fixup_pointer(early_dynamic_pgts[next_early_pgt++], physaddr);
> -       pmd = fixup_pointer(early_dynamic_pgts[next_early_pgt++], physaddr);
> +       next_pgt_ptr = fixup_pointer(&next_early_pgt, physaddr);
> +       pud = fixup_pointer(early_dynamic_pgts[(*next_pgt_ptr)++], physaddr);
> +       pmd = fixup_pointer(early_dynamic_pgts[(*next_pgt_ptr)++], physaddr);
>
>         if (IS_ENABLED(CONFIG_X86_5LEVEL)) {
>                 p4d = fixup_pointer(early_dynamic_pgts[next_early_pgt++], physaddr);
> --
> 2.14.1.480.gb18f417b89-goog
>



-- 
Alexander Potapenko
Software Engineer

Google Germany GmbH
Erika-Mann-Straße, 33
80636 München

Geschäftsführer: Paul Manicle, Halimah DeLaine Prado
Registergericht und -nummer: Hamburg, HRB 86891
Sitz der Gesellschaft: Hamburg

Re: [PATCH] x86/boot/64: clang: use fixup_pointer() to access next_early_pgt

[Kirill A. Shutemov]

On Wed, Aug 16, 2017 at 09:08:08PM +0200, Alexander Potapenko wrote:
> __startup_64() is normally using fixup_pointer() to access globals in a
> position-independent fashion. However |next_early_pgt| was accessed
> directly, which wasn't guaranteed to work.
> 
> Luckily GCC was generating a R_X86_64_PC32 PC-relative relocation for
> |next_early_pgt|, but Clang emitted a R_X86_64_32S, which led to
> accessing invalid memory and rebooting the kernel.

Thanks for tracking this down.

Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>

> Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> Cc: Dmitry Vyukov <dvyukov@google.com>
> Cc: Michael Davidson <md@google.com>
> Fixes: c88d71508e36 ("x86/boot/64: Rewrite startup_64() in C")

Cc: <stable@vger.kernel.org> # 4.12

-- 
 Kirill A. Shutemov

[tip:x86/urgent] x86/boot/64/clang: Use fixup_pointer() to access 'next_early_pgt'

[tip-bot for Alexander Potapenko]

Commit-ID:  187e91fe5e915f4b7f39b824aa422493463e443d
Gitweb:     http://git.kernel.org/tip/187e91fe5e915f4b7f39b824aa422493463e443d
Author:     Alexander Potapenko <glider@google.com>
AuthorDate: Wed, 16 Aug 2017 21:08:08 +0200
Committer:  Ingo Molnar <mingo@kernel.org>
CommitDate: Thu, 17 Aug 2017 09:53:00 +0200

x86/boot/64/clang: Use fixup_pointer() to access 'next_early_pgt'

__startup_64() is normally using fixup_pointer() to access globals in a
position-independent fashion. However 'next_early_pgt' was accessed
directly, which wasn't guaranteed to work.

Luckily GCC was generating a R_X86_64_PC32 PC-relative relocation for
'next_early_pgt', but Clang emitted a R_X86_64_32S, which led to
accessing invalid memory and rebooting the kernel.

Signed-off-by: Alexander Potapenko <glider@google.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Michael Davidson <md@google.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Fixes: c88d71508e36 ("x86/boot/64: Rewrite startup_64() in C")
Link: http://lkml.kernel.org/r/20170816190808.131748-1-glider@google.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 arch/x86/kernel/head64.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kernel/head64.c b/arch/x86/kernel/head64.c
index 46c3c73..9ba7954 100644
--- a/arch/x86/kernel/head64.c
+++ b/arch/x86/kernel/head64.c
@@ -53,6 +53,7 @@ void __head __startup_64(unsigned long physaddr)
 	pudval_t *pud;
 	pmdval_t *pmd, pmd_entry;
 	int i;
+	unsigned int *next_pgt_ptr;
 
 	/* Is the address too large? */
 	if (physaddr >> MAX_PHYSMEM_BITS)
@@ -91,9 +92,9 @@ void __head __startup_64(unsigned long physaddr)
 	 * creates a bunch of nonsense entries but that is fine --
 	 * it avoids problems around wraparound.
 	 */
-
-	pud = fixup_pointer(early_dynamic_pgts[next_early_pgt++], physaddr);
-	pmd = fixup_pointer(early_dynamic_pgts[next_early_pgt++], physaddr);
+	next_pgt_ptr = fixup_pointer(&next_early_pgt, physaddr);
+	pud = fixup_pointer(early_dynamic_pgts[(*next_pgt_ptr)++], physaddr);
+	pmd = fixup_pointer(early_dynamic_pgts[(*next_pgt_ptr)++], physaddr);
 
 	if (IS_ENABLED(CONFIG_X86_5LEVEL)) {
 		p4d = fixup_pointer(early_dynamic_pgts[next_early_pgt++], physaddr);