diff mbox series

[v2,1/1] x86/mm/KASLR: Store pud_page_tramp into entry rather than page

Message ID 20230614163859.924309-2-lee@kernel.org
State New
Headers
Series x86: Fix .bss corruption |

Commit Message

Lee Jones June 14, 2023, 4:38 p.m. UTC 
  set_pgd() expects to be passed whole pages to operate on, whereas
trampoline_pgd_entry is, as the name suggests, an entry.  The
ramifications for using set_pgd() here are that the following thread of
execution will not only place the suggested value into the
trampoline_pgd_entry (8-Byte globally stored [.bss]) variable, PTI will
also attempt to replicate that value into the non-existent neighboring
user page (located +4k away), leading to the corruption of other global
[.bss] stored variables.

Suggested-by: Dave Hansen <dave.hansen@linux.intel.com>
Signed-off-by: Lee Jones <lee@kernel.org>
---
 arch/x86/mm/kaslr.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

Comments

Nick Desaulniers June 14, 2023, 5:48 p.m. UTC | #1 
On Wed, Jun 14, 2023 at 05:38:54PM +0100, Lee Jones wrote:
> set_pgd() expects to be passed whole pages to operate on, whereas
> trampoline_pgd_entry is, as the name suggests, an entry.  The
> ramifications for using set_pgd() here are that the following thread of
> execution will not only place the suggested value into the
> trampoline_pgd_entry (8-Byte globally stored [.bss]) variable, PTI will
> also attempt to replicate that value into the non-existent neighboring
> user page (located +4k away), leading to the corruption of other global
> [.bss] stored variables.
> 
> Suggested-by: Dave Hansen <dave.hansen@linux.intel.com>
> Signed-off-by: Lee Jones <lee@kernel.org>

Nice work tracking this one down!

Fixes: 0925dda5962e ("x86/mm/KASLR: Use only one PUD entry for real mode trampoline")
Cc: <stable@vger.kernel.org>

> ---
>  arch/x86/mm/kaslr.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/mm/kaslr.c b/arch/x86/mm/kaslr.c
> index 557f0fe25dff4..37db264866b64 100644
> --- a/arch/x86/mm/kaslr.c
> +++ b/arch/x86/mm/kaslr.c
> @@ -172,10 +172,10 @@ void __meminit init_trampoline_kaslr(void)
>  		set_p4d(p4d_tramp,
>  			__p4d(_KERNPG_TABLE | __pa(pud_page_tramp)));
>  
> -		set_pgd(&trampoline_pgd_entry,
> -			__pgd(_KERNPG_TABLE | __pa(p4d_page_tramp)));
> +		trampoline_pgd_entry =
> +			__pgd(_KERNPG_TABLE | __pa(p4d_page_tramp));
>  	} else {
> -		set_pgd(&trampoline_pgd_entry,
> -			__pgd(_KERNPG_TABLE | __pa(pud_page_tramp)));
> +		trampoline_pgd_entry =
> +			__pgd(_KERNPG_TABLE | __pa(pud_page_tramp));
>  	}
>  }
> -- 
> 2.41.0.162.gfafddb0af9-goog
>
Lee Jones June 14, 2023, 6:43 p.m. UTC | #2 
On Wed, 14 Jun 2023, Nick Desaulniers wrote:

> On Wed, Jun 14, 2023 at 05:38:54PM +0100, Lee Jones wrote:
> > set_pgd() expects to be passed whole pages to operate on, whereas
> > trampoline_pgd_entry is, as the name suggests, an entry.  The
> > ramifications for using set_pgd() here are that the following thread of
> > execution will not only place the suggested value into the
> > trampoline_pgd_entry (8-Byte globally stored [.bss]) variable, PTI will
> > also attempt to replicate that value into the non-existent neighboring
> > user page (located +4k away), leading to the corruption of other global
> > [.bss] stored variables.
> > 
> > Suggested-by: Dave Hansen <dave.hansen@linux.intel.com>
> > Signed-off-by: Lee Jones <lee@kernel.org>
> 
> Nice work tracking this one down!
> 
> Fixes: 0925dda5962e ("x86/mm/KASLR: Use only one PUD entry for real mode trampoline")
> Cc: <stable@vger.kernel.org>

I was planning on backporting this myself once it hits Mainline.

Happy to submit a RESEND with those pieces added if required though.

Or perhaps someone would be kind enough to add them on merge?

> > ---
> >  arch/x86/mm/kaslr.c | 8 ++++----
> >  1 file changed, 4 insertions(+), 4 deletions(-)
> > 
> > diff --git a/arch/x86/mm/kaslr.c b/arch/x86/mm/kaslr.c
> > index 557f0fe25dff4..37db264866b64 100644
> > --- a/arch/x86/mm/kaslr.c
> > +++ b/arch/x86/mm/kaslr.c
> > @@ -172,10 +172,10 @@ void __meminit init_trampoline_kaslr(void)
> >  		set_p4d(p4d_tramp,
> >  			__p4d(_KERNPG_TABLE | __pa(pud_page_tramp)));
> >  
> > -		set_pgd(&trampoline_pgd_entry,
> > -			__pgd(_KERNPG_TABLE | __pa(p4d_page_tramp)));
> > +		trampoline_pgd_entry =
> > +			__pgd(_KERNPG_TABLE | __pa(p4d_page_tramp));
> >  	} else {
> > -		set_pgd(&trampoline_pgd_entry,
> > -			__pgd(_KERNPG_TABLE | __pa(pud_page_tramp)));
> > +		trampoline_pgd_entry =
> > +			__pgd(_KERNPG_TABLE | __pa(pud_page_tramp));
> >  	}
> >  }
> > -- 
> > 2.41.0.162.gfafddb0af9-goog
> >
Lee Jones June 16, 2023, 5 p.m. UTC | #3 
On Wed, 14 Jun 2023, Lee Jones wrote:

> set_pgd() expects to be passed whole pages to operate on, whereas
> trampoline_pgd_entry is, as the name suggests, an entry.  The
> ramifications for using set_pgd() here are that the following thread of
> execution will not only place the suggested value into the
> trampoline_pgd_entry (8-Byte globally stored [.bss]) variable, PTI will
> also attempt to replicate that value into the non-existent neighboring
> user page (located +4k away), leading to the corruption of other global
> [.bss] stored variables.
> 
> Suggested-by: Dave Hansen <dave.hansen@linux.intel.com>
> Signed-off-by: Lee Jones <lee@kernel.org>
> ---
>  arch/x86/mm/kaslr.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)

Is there any more you need from me at this point?

Would you like me to resubmit with the Fixes: tag applied, or is
someone happy to apply it on merge?

> diff --git a/arch/x86/mm/kaslr.c b/arch/x86/mm/kaslr.c
> index 557f0fe25dff4..37db264866b64 100644
> --- a/arch/x86/mm/kaslr.c
> +++ b/arch/x86/mm/kaslr.c
> @@ -172,10 +172,10 @@ void __meminit init_trampoline_kaslr(void)
>  		set_p4d(p4d_tramp,
>  			__p4d(_KERNPG_TABLE | __pa(pud_page_tramp)));
>  
> -		set_pgd(&trampoline_pgd_entry,
> -			__pgd(_KERNPG_TABLE | __pa(p4d_page_tramp)));
> +		trampoline_pgd_entry =
> +			__pgd(_KERNPG_TABLE | __pa(p4d_page_tramp));
>  	} else {
> -		set_pgd(&trampoline_pgd_entry,
> -			__pgd(_KERNPG_TABLE | __pa(pud_page_tramp)));
> +		trampoline_pgd_entry =
> +			__pgd(_KERNPG_TABLE | __pa(pud_page_tramp));
>  	}
>  }
> -- 
> 2.41.0.162.gfafddb0af9-goog
>
Lee Jones June 19, 2023, 10:43 a.m. UTC | #4 
On Fri, 16 Jun 2023, Lee Jones wrote:

> On Wed, 14 Jun 2023, Lee Jones wrote:
> 
> > set_pgd() expects to be passed whole pages to operate on, whereas
> > trampoline_pgd_entry is, as the name suggests, an entry.  The
> > ramifications for using set_pgd() here are that the following thread of
> > execution will not only place the suggested value into the
> > trampoline_pgd_entry (8-Byte globally stored [.bss]) variable, PTI will
> > also attempt to replicate that value into the non-existent neighboring
> > user page (located +4k away), leading to the corruption of other global
> > [.bss] stored variables.
> > 
> > Suggested-by: Dave Hansen <dave.hansen@linux.intel.com>
> > Signed-off-by: Lee Jones <lee@kernel.org>
> > ---
> >  arch/x86/mm/kaslr.c | 8 ++++----
> >  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> Is there any more you need from me at this point?
> 
> Would you like me to resubmit with the Fixes: tag applied, or is
> someone happy to apply it on merge?

Superstar, thanks Dave.
diff mbox series

Patch

diff --git a/arch/x86/mm/kaslr.c b/arch/x86/mm/kaslr.c
index 557f0fe25dff4..37db264866b64 100644
--- a/arch/x86/mm/kaslr.c
+++ b/arch/x86/mm/kaslr.c
@@ -172,10 +172,10 @@  void __meminit init_trampoline_kaslr(void)
 		set_p4d(p4d_tramp,
 			__p4d(_KERNPG_TABLE | __pa(pud_page_tramp)));
 
-		set_pgd(&trampoline_pgd_entry,
-			__pgd(_KERNPG_TABLE | __pa(p4d_page_tramp)));
+		trampoline_pgd_entry =
+			__pgd(_KERNPG_TABLE | __pa(p4d_page_tramp));
 	} else {
-		set_pgd(&trampoline_pgd_entry,
-			__pgd(_KERNPG_TABLE | __pa(pud_page_tramp)));
+		trampoline_pgd_entry =
+			__pgd(_KERNPG_TABLE | __pa(pud_page_tramp));
 	}
 }