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SubjectRe: [PATCH v2 4/5] x86/boot/64: Explicitly map boot_params and command line
On Thu, Oct 08, 2020 at 03:16:22PM -0400, Arvind Sankar wrote:
> Commits
>
> ca0e22d4f011 ("x86/boot/compressed/64: Always switch to own page table")
> 8570978ea030 ("x86/boot/compressed/64: Don't pre-map memory in KASLR code")
>
> set up a new page table in the decompressor stub, but without explicit
> mappings for boot_params and the kernel command line, relying on the #PF
> handler instead.
>
> This is fragile, as boot_params and the command line mappings are
> required for the main kernel. If EARLY_PRINTK and RANDOMIZE_BASE are
> disabled, a QEMU/OVMF boot never accesses the command line in the
> decompressor stub, and so it never gets mapped. The main kernel accesses
> it from the identity mapping if AMD_MEM_ENCRYPT is enabled, and will
> crash.
>
> Fix this by adding back the explicit mapping of boot_params and the
> command line.
>
> Note: the changes also removed the explicit mapping of the main kernel,
> with the result that .bss and .brk may not be in the identity mapping,
> but those don't get accessed by the main kernel before it switches to
> its own page tables.
>
> Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
> ---
> arch/x86/boot/compressed/ident_map_64.c | 16 ++++++++++++++--
> 1 file changed, 14 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/boot/compressed/ident_map_64.c b/arch/x86/boot/compressed/ident_map_64.c
> index fd957b2625e9..a3613857c532 100644
> --- a/arch/x86/boot/compressed/ident_map_64.c
> +++ b/arch/x86/boot/compressed/ident_map_64.c
> @@ -21,6 +21,7 @@
>
> #include "error.h"
> #include "misc.h"
> +#include "cmdline.h"
>
> /* These actually do the work of building the kernel identity maps. */
> #include <linux/pgtable.h>
> @@ -109,6 +110,8 @@ static void add_identity_map(unsigned long start, unsigned long end)
> /* Locates and clears a region for a new top level page table. */
> void initialize_identity_maps(void)
> {
> + unsigned long cmdline;
> +
> /* Exclude the encryption mask from __PHYSICAL_MASK */
> physical_mask &= ~sme_me_mask;
>
> @@ -149,10 +152,19 @@ void initialize_identity_maps(void)
> }
>
> /*
> - * New page-table is set up - map the kernel image and load it
> - * into cr3.
> + * New page-table is set up - map the kernel image, boot_params and the
> + * command line.
> + * The uncompressed kernel requires boot_params and the command line to
> + * be mapped in the identity mapping.
> + * Map them explicitly here in case the compressed kernel does not
> + * touch them, or does not touch all the pages covering them.
> */
> add_identity_map((unsigned long)_head, (unsigned long)_end);
> + add_identity_map((unsigned long)boot_params, (unsigned long)(boot_params + 1));
> + cmdline = get_cmd_line_ptr();
> + add_identity_map(cmdline, cmdline + COMMAND_LINE_SIZE);
> +
> + /* Load the new page-table. */
> write_cr3(top_level_pgt);
> }

Reviewed-by: Joerg Roedel <jroedel@suse.de>

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