2019-09-02 10:16:40

by Zhenzhong Duan

Subject: [PATCH] x86/boot/compressed/64: Fix 'may be used uninitialized' issue

Builds kernel with "make bzImage EXTRA_CFLAGS=-Wall", gcc complains:

arch/x86/boot/compressed/pgtable_64.c: In function 'paging_prepare':
arch/x86/boot/compressed/pgtable_64.c:92:7: warning: 'new' may be used uninitialized in this function [-Wmaybe-uninitialized]
new = round_down(new, PAGE_SIZE);

In theory a random value of variable new may pass all the check and be
assigned to bios_start, fixing it by assigning it an initial value.

Fixes: 0a46fff2f910 ("x86/boot/compressed/64: Fix boot on machines with broken E820 table")
Signed-off-by: Zhenzhong Duan <[email protected]>
Cc: Kirill A. Shutemov <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: "H. Peter Anvin" <[email protected]>
Cc: x86-ml <[email protected]>
---
arch/x86/boot/compressed/pgtable_64.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/boot/compressed/pgtable_64.c b/arch/x86/boot/compressed/pgtable_64.c
index 2faddeb..c886269 100644
--- a/arch/x86/boot/compressed/pgtable_64.c
+++ b/arch/x86/boot/compressed/pgtable_64.c
@@ -72,7 +72,7 @@ static unsigned long find_trampoline_placement(void)

/* Find the first usable memory region under bios_start. */
for (i = boot_params->e820_entries - 1; i >= 0; i--) {
- unsigned long new;
+ unsigned long new = bios_start;

entry = &boot_params->e820_table[i];

--
1.8.3.1

2019-09-02 10:16:56

by Borislav Petkov

Subject: Re: [PATCH] x86/boot/compressed/64: Fix 'may be used uninitialized' issue

On Mon, Sep 02, 2019 at 05:30:24PM +0800, Zhenzhong Duan wrote:
> Builds kernel with "make bzImage EXTRA_CFLAGS=-Wall", gcc complains:
>
> arch/x86/boot/compressed/pgtable_64.c: In function 'paging_prepare':
> arch/x86/boot/compressed/pgtable_64.c:92:7: warning: 'new' may be used uninitialized in this function [-Wmaybe-uninitialized]
> new = round_down(new, PAGE_SIZE);
>
> In theory a random value of variable new may pass all the check and be
> assigned to bios_start, fixing it by assigning it an initial value.
>
> Fixes: 0a46fff2f910 ("x86/boot/compressed/64: Fix boot on machines with broken E820 table")
> Signed-off-by: Zhenzhong Duan <[email protected]>
> Cc: Kirill A. Shutemov <[email protected]>
> Cc: Thomas Gleixner <[email protected]>
> Cc: Ingo Molnar <[email protected]>
> Cc: Borislav Petkov <[email protected]>
> Cc: "H. Peter Anvin" <[email protected]>
> Cc: x86-ml <[email protected]>
> ---
> arch/x86/boot/compressed/pgtable_64.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/x86/boot/compressed/pgtable_64.c b/arch/x86/boot/compressed/pgtable_64.c
> index 2faddeb..c886269 100644
> --- a/arch/x86/boot/compressed/pgtable_64.c
> +++ b/arch/x86/boot/compressed/pgtable_64.c
> @@ -72,7 +72,7 @@ static unsigned long find_trampoline_placement(void)
>
> /* Find the first usable memory region under bios_start. */
> for (i = boot_params->e820_entries - 1; i >= 0; i--) {
> - unsigned long new;
> + unsigned long new = bios_start;
>
> entry = &boot_params->e820_table[i];
>
> --

https://git.kernel.org/tip/c96e8483cb2da6695c8b8d0896fe7ae272a07b54

In the future, use tip/master when preparing fixes because it has the
latest state of the x86 tree and you won't have to duplicate effort,
like in this case.

Thx.

--
Regards/Gruss,
Boris.

Good mailing practices for 400: avoid top-posting and trim the reply.

2019-09-02 10:17:12

by Zhenzhong Duan

Subject: Re: [PATCH] x86/boot/compressed/64: Fix 'may be used uninitialized' issue

On 2019/9/2 17:35, Borislav Petkov wrote:
> On Mon, Sep 02, 2019 at 05:30:24PM +0800, Zhenzhong Duan wrote:
>> Builds kernel with "make bzImage EXTRA_CFLAGS=-Wall", gcc complains:
>>
>> arch/x86/boot/compressed/pgtable_64.c: In function 'paging_prepare':
>> arch/x86/boot/compressed/pgtable_64.c:92:7: warning: 'new' may be used uninitialized in this function [-Wmaybe-uninitialized]
>> new = round_down(new, PAGE_SIZE);
>>
>> In theory a random value of variable new may pass all the check and be
>> assigned to bios_start, fixing it by assigning it an initial value.
>>
>> Fixes: 0a46fff2f910 ("x86/boot/compressed/64: Fix boot on machines with broken E820 table")
>> Signed-off-by: Zhenzhong Duan<[email protected]>
>> Cc: Kirill A. Shutemov<[email protected]>
>> Cc: Thomas Gleixner<[email protected]>
>> Cc: Ingo Molnar<[email protected]>
>> Cc: Borislav Petkov<[email protected]>
>> Cc: "H. Peter Anvin"<[email protected]>
>> Cc: x86-ml<[email protected]>
>> ---
>> arch/x86/boot/compressed/pgtable_64.c | 2 +-
>> 1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/arch/x86/boot/compressed/pgtable_64.c b/arch/x86/boot/compressed/pgtable_64.c
>> index 2faddeb..c886269 100644
>> --- a/arch/x86/boot/compressed/pgtable_64.c
>> +++ b/arch/x86/boot/compressed/pgtable_64.c
>> @@ -72,7 +72,7 @@ static unsigned long find_trampoline_placement(void)
>>
>> /* Find the first usable memory region under bios_start. */
>> for (i = boot_params->e820_entries - 1; i >= 0; i--) {
>> - unsigned long new;
>> + unsigned long new = bios_start;
>>
>> entry = &boot_params->e820_table[i];
>>
>> --
> https://git.kernel.org/tip/c96e8483cb2da6695c8b8d0896fe7ae272a07b54
>
> In the future, use tip/master when preparing fixes because it has the
> latest state of the x86 tree and you won't have to duplicate effort,
> like in this case.

Ok, I see. Thanks for your suggestion.

Zhenzhong