[PATCH] riscv: mm: Proper page permissions after initmem free

[PATCH] riscv: mm: Proper page permissions after initmem free

[Björn Töpel]

From: Björn Töpel <bjorn@rivosinc.com>

64-bit RISC-V kernels have the kernel image mapped separately, and in
addition to the linear map. When the kernel is loaded, the linear map
of kernel image is set to PAGE_READ permission, and the kernel map is
set to PAGE_READ and PAGE_EXEC.

When the initmem is freed, the corresponding pages in the linear map
should be restored to PAGE_READ and PAGE_WRITE. The corresponding
pages in the kernel map should also be restored to PAGE_READ and
PAGE_WRITE, by removing the PAGE_EXEC permission, and adding
PAGE_WRITE.

This is not the case. For 64-bit kernels, only the linear map is
restored to its proper page permissions at initmem free, and not the
kernelmap.

In practise this results in that the kernel can potentially jump to
dead __init code, and start executing invalid 0xcc instructions,
without getting an exception.

Restore the freed initmem properly, by setting both the alias (kernel
map) and the linear map to the correct permissions.

Fixes: e5c35fa04019 ("riscv: Map the kernel with correct permissions the first time")
Signed-off-by: Björn Töpel <bjorn@rivosinc.com>
---
 arch/riscv/kernel/setup.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/arch/riscv/kernel/setup.c b/arch/riscv/kernel/setup.c
index ad76bb59b059..361e635070fe 100644
--- a/arch/riscv/kernel/setup.c
+++ b/arch/riscv/kernel/setup.c
@@ -321,10 +321,12 @@ subsys_initcall(topology_init);
 
 void free_initmem(void)
 {
-	if (IS_ENABLED(CONFIG_STRICT_KERNEL_RWX))
-		set_kernel_memory(lm_alias(__init_begin), lm_alias(__init_end),
-				  IS_ENABLED(CONFIG_64BIT) ?
-					set_memory_rw : set_memory_rw_nx);
+	if (IS_ENABLED(CONFIG_STRICT_KERNEL_RWX)) {
+		if (IS_ENABLED(CONFIG_64BIT))
+			set_kernel_memory(lm_alias(__init_begin), lm_alias(__init_end),
+					  set_memory_rw);
+		set_kernel_memory(__init_begin, __init_end, set_memory_rw_nx);
+	}
 
 	free_initmem_default(POISON_FREE_INITMEM);
 }

base-commit: 442bcbfd2c5401587b983e34bed0b407214735c3
-- 
2.37.2

Re: [PATCH] riscv: mm: Proper page permissions after initmem free

[Samuel Holland]

On 11/12/22 05:35, Björn Töpel wrote:
> From: Björn Töpel <bjorn@rivosinc.com>
> 
> 64-bit RISC-V kernels have the kernel image mapped separately, and in
> addition to the linear map. When the kernel is loaded, the linear map
> of kernel image is set to PAGE_READ permission, and the kernel map is
> set to PAGE_READ and PAGE_EXEC.
> 
> When the initmem is freed, the corresponding pages in the linear map
> should be restored to PAGE_READ and PAGE_WRITE. The corresponding
> pages in the kernel map should also be restored to PAGE_READ and
> PAGE_WRITE, by removing the PAGE_EXEC permission, and adding
> PAGE_WRITE.
> 
> This is not the case. For 64-bit kernels, only the linear map is
> restored to its proper page permissions at initmem free, and not the
> kernelmap.
> 
> In practise this results in that the kernel can potentially jump to
> dead __init code, and start executing invalid 0xcc instructions,
> without getting an exception.
> 
> Restore the freed initmem properly, by setting both the alias (kernel
> map) and the linear map to the correct permissions.
> 
> Fixes: e5c35fa04019 ("riscv: Map the kernel with correct permissions the first time")
> Signed-off-by: Björn Töpel <bjorn@rivosinc.com>
> ---
>  arch/riscv/kernel/setup.c | 10 ++++++----
>  1 file changed, 6 insertions(+), 4 deletions(-)

Reviewed-by: Samuel Holland <samuel@sholland.org>
Tested-by: Samuel Holland <samuel@sholland.org> # on D1

Re: [PATCH] riscv: mm: Proper page permissions after initmem free

[Alexandre Ghiti]

Hi Björn,

On 12/11/2022 12:35, Björn Töpel wrote:
> From: Björn Töpel <bjorn@rivosinc.com>
>
> 64-bit RISC-V kernels have the kernel image mapped separately, and in
> addition to the linear map. When the kernel is loaded, the linear map
> of kernel image is set to PAGE_READ permission, and the kernel map is
> set to PAGE_READ and PAGE_EXEC.
>
> When the initmem is freed, the corresponding pages in the linear map
> should be restored to PAGE_READ and PAGE_WRITE. The corresponding
> pages in the kernel map should also be restored to PAGE_READ and
> PAGE_WRITE, by removing the PAGE_EXEC permission, and adding
> PAGE_WRITE.
>
> This is not the case. For 64-bit kernels, only the linear map is
> restored to its proper page permissions at initmem free, and not the
> kernelmap.
>
> In practise this results in that the kernel can potentially jump to
> dead __init code, and start executing invalid 0xcc instructions,
> without getting an exception.
>
> Restore the freed initmem properly, by setting both the alias (kernel
> map) and the linear map to the correct permissions.
>
> Fixes: e5c35fa04019 ("riscv: Map the kernel with correct permissions the first time")
> Signed-off-by: Björn Töpel <bjorn@rivosinc.com>
> ---
>   arch/riscv/kernel/setup.c | 10 ++++++----
>   1 file changed, 6 insertions(+), 4 deletions(-)
>
> diff --git a/arch/riscv/kernel/setup.c b/arch/riscv/kernel/setup.c
> index ad76bb59b059..361e635070fe 100644
> --- a/arch/riscv/kernel/setup.c
> +++ b/arch/riscv/kernel/setup.c
> @@ -321,10 +321,12 @@ subsys_initcall(topology_init);
>   
>   void free_initmem(void)
>   {
> -	if (IS_ENABLED(CONFIG_STRICT_KERNEL_RWX))
> -		set_kernel_memory(lm_alias(__init_begin), lm_alias(__init_end),
> -				  IS_ENABLED(CONFIG_64BIT) ?
> -					set_memory_rw : set_memory_rw_nx);
> +	if (IS_ENABLED(CONFIG_STRICT_KERNEL_RWX)) {
> +		if (IS_ENABLED(CONFIG_64BIT))
> +			set_kernel_memory(lm_alias(__init_begin), lm_alias(__init_end),
> +					  set_memory_rw);
> +		set_kernel_memory(__init_begin, __init_end, set_memory_rw_nx);


That's nits but for 64-bit kernels, do we really want to make the kernel 
mapping writable? I think catching a write access here would be better 
because IMO that should not happen.

Thanks,

Alex


> +	}
>   
>   	free_initmem_default(POISON_FREE_INITMEM);
>   }
>
> base-commit: 442bcbfd2c5401587b983e34bed0b407214735c3

Re: [PATCH] riscv: mm: Proper page permissions after initmem free

[Björn Töpel]

Alexandre Ghiti <alex@ghiti.fr> writes:

>> +		set_kernel_memory(__init_begin, __init_end, set_memory_rw_nx);
>
>
> That's nits but for 64-bit kernels, do we really want to make the kernel 
> mapping writable? I think catching a write access here would be better 
> because IMO that should not happen.

Thanks for having a look!

Hmm, so what you're suggesting is a "set_memory_r_nx" for CONFIG_64BIT?
What do other archs do? Is it worth the extra ifdef clutter?


Björn