| Message ID | 63368ff665956a64f07aee9bc863b70c86b8b0c8.1678785672.git.baskov@ispras.ru |
|---|---|
| State | New |
| Headers | show |
| Series | x86_64: Improvements at compressed kernel stage | expand |
On Tue, Mar 14, 2023 at 01:13:30PM +0300, Evgeniy Baskov wrote: > Ensure WP bit to be set to prevent boot code from writing to > non-writable memory pages. > > Tested-by: Mario Limonciello <mario.limonciello@amd.com> > Signed-off-by: Evgeniy Baskov <baskov@ispras.ru> > --- > arch/x86/boot/compressed/head_64.S | 5 ++--- > 1 file changed, 2 insertions(+), 3 deletions(-) > > diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S > index 03c4328a88cb..01fa42d31648 100644 > --- a/arch/x86/boot/compressed/head_64.S > +++ b/arch/x86/boot/compressed/head_64.S > @@ -660,9 +660,8 @@ SYM_CODE_START(trampoline_32bit_src) > pushl $__KERNEL_CS > pushl %eax > > - /* Enable paging again. */ > - movl %cr0, %eax > - btsl $X86_CR0_PG_BIT, %eax > + /* Enable paging and set CR0 to known state (this also sets WP flag) */ > + movl $CR0_STATE, %eax This sets a lot more than WP. Why?
On 2023-04-05 20:54, Borislav Petkov wrote: > On Tue, Mar 14, 2023 at 01:13:30PM +0300, Evgeniy Baskov wrote: >> Ensure WP bit to be set to prevent boot code from writing to >> non-writable memory pages. >> >> Tested-by: Mario Limonciello <mario.limonciello@amd.com> >> Signed-off-by: Evgeniy Baskov <baskov@ispras.ru> >> --- >> arch/x86/boot/compressed/head_64.S | 5 ++--- >> 1 file changed, 2 insertions(+), 3 deletions(-) >> >> diff --git a/arch/x86/boot/compressed/head_64.S >> b/arch/x86/boot/compressed/head_64.S >> index 03c4328a88cb..01fa42d31648 100644 >> --- a/arch/x86/boot/compressed/head_64.S >> +++ b/arch/x86/boot/compressed/head_64.S >> @@ -660,9 +660,8 @@ SYM_CODE_START(trampoline_32bit_src) >> pushl $__KERNEL_CS >> pushl %eax >> >> - /* Enable paging again. */ >> - movl %cr0, %eax >> - btsl $X86_CR0_PG_BIT, %eax >> + /* Enable paging and set CR0 to known state (this also sets WP flag) >> */ >> + movl $CR0_STATE, %eax > > This sets a lot more than WP. Why? Because there are code paths where cr0 state is not initialized (e.g. the EFISTUB code path) and it's better to know it exactly. Although we don't actually care about MP, ET, NE and AM flags, but they should be all supported, so the choice was arbitrary. Also they are already initialized to this value on one code path -- when the kernel started its execution via startup_32. Thanks.
diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S index 03c4328a88cb..01fa42d31648 100644 --- a/arch/x86/boot/compressed/head_64.S +++ b/arch/x86/boot/compressed/head_64.S @@ -660,9 +660,8 @@ SYM_CODE_START(trampoline_32bit_src) pushl $__KERNEL_CS pushl %eax - /* Enable paging again. */ - movl %cr0, %eax - btsl $X86_CR0_PG_BIT, %eax + /* Enable paging and set CR0 to known state (this also sets WP flag) */ + movl $CR0_STATE, %eax movl %eax, %cr0 lret