Re: [PATCHv4 4/8] x86/boot/compressed: Handle unaccepted memory

Kirill A. Shutemov
2022-04-09T20:19:09Z

On Fri, Apr 08, 2022 at 10:57:17AM -0700, Dave Hansen wrote:
> On 4/5/22 16:43, Kirill A. Shutemov wrote:
> > Firmware is responsible for accepting memory where compressed kernel
> > image and initrd land. But kernel has to accept memory for decompression
> > buffer: accept memory just before decompression starts.
>
> I think I'd appreciate a sentence or two more about what's going on.
> How about something like this?
>
> The firmware starts the kernel by booting into the "compressed" kernel
> stub. That stub's job is to decompress the full kernel image and then
> jump to it.
>
> The firmware will pre-accept the memory used to run the stub. But, the
> stub is responsible for accepting the memory into which it decompresses
> the main kernel. Accept memory just before decompression starts.
>
> The stub is also responsible for choosing a physical address in which to
> place the decompressed kernel image. The KASLR mechanism will randomize
> this physical address. Since the unaccepted memory region is relatively
> small, KASLR would be quite ineffective if it only used the pre-accepted
> area (EFI_CONVENTIONAL_MEMORY). Ensure that KASLR randomizes among the
> entire physical address space by also including EFI_UNACCEPTED_MEMORY.

Sure, looks good.

> > diff --git a/arch/x86/boot/compressed/bitmap.c b/arch/x86/boot/compressed/bitmap.c
> > index bf58b259380a..ba2de61c0823 100644
> > --- a/arch/x86/boot/compressed/bitmap.c
> > +++ b/arch/x86/boot/compressed/bitmap.c
> > @@ -2,6 +2,48 @@
> > /* Taken from lib/string.c */
> >
> > #include <linux/bitmap.h>
> > +#include <linux/math.h>
> > +#include <linux/minmax.h>
> > +
> > +unsigned long _find_next_bit(const unsigned long *addr1,
> > + const unsigned long *addr2, unsigned long nbits,
> > + unsigned long start, unsigned long invert, unsigned long le)
> > +{
> > + unsigned long tmp, mask;
> > +
> > + if (unlikely(start >= nbits))
> > + return nbits;
> > +
> > + tmp = addr1[start / BITS_PER_LONG];
> > + if (addr2)
> > + tmp &= addr2[start / BITS_PER_LONG];
> > + tmp ^= invert;
> > +
> > + /* Handle 1st word. */
> > + mask = BITMAP_FIRST_WORD_MASK(start);
> > + if (le)
> > + mask = swab(mask);
> > +
> > + tmp &= mask;
> > +
> > + start = round_down(start, BITS_PER_LONG);
> > +
> > + while (!tmp) {
> > + start += BITS_PER_LONG;
> > + if (start >= nbits)
> > + return nbits;
> > +
> > + tmp = addr1[start / BITS_PER_LONG];
> > + if (addr2)
> > + tmp &= addr2[start / BITS_PER_LONG];
> > + tmp ^= invert;
> > + }
> > +
> > + if (le)
> > + tmp = swab(tmp);
> > +
> > + return min(start + __ffs(tmp), nbits);
> > +}
> >
> > void __bitmap_set(unsigned long *map, unsigned int start, int len)
> > {
> > @@ -22,3 +64,23 @@ void __bitmap_set(unsigned long *map, unsigned int start, int len)
> > *p |= mask_to_set;
> > }
> > }
> > +
> > +void __bitmap_clear(unsigned long *map, unsigned int start, int len)
> > +{
> > + unsigned long *p = map + BIT_WORD(start);
> > + const unsigned int size = start + len;
> > + int bits_to_clear = BITS_PER_LONG - (start % BITS_PER_LONG);
> > + unsigned long mask_to_clear = BITMAP_FIRST_WORD_MASK(start);
> > +
> > + while (len - bits_to_clear >= 0) {
> > + *p &= ~mask_to_clear;
> > + len -= bits_to_clear;
> > + bits_to_clear = BITS_PER_LONG;
> > + mask_to_clear = ~0UL;
> > + p++;
> > + }
> > + if (len) {
> > + mask_to_clear &= BITMAP_LAST_WORD_MASK(size);
> > + *p &= ~mask_to_clear;
> > + }
> > +}
>
> It's a real shame that we have to duplicate this code. Is there
> anything crazy we could do here like
>
> #include "../../../lib/find_bit.c"
>
> ?

Well, it would require fracturing source files on the kernel side.

__bitmap_set() and __bitmap_clear() are now in lib/bitmap.c.

_find_next_bit() is in lib/find_bit.c.

Both lib/bitmap.c and lib/find_bit.c have a lot of stuff that are not used
here. I guess we would need to split them into few pieces to make it in
sane way. Do you want me to go this path?

>
> > diff --git a/arch/x86/boot/compressed/kaslr.c b/arch/x86/boot/compressed/kaslr.c
> > index 411b268bc0a2..59db90626042 100644
> > --- a/arch/x86/boot/compressed/kaslr.c
> > +++ b/arch/x86/boot/compressed/kaslr.c
> > @@ -725,10 +725,20 @@ process_efi_entries(unsigned long minimum, unsigned long image_size)
> > * but in practice there's firmware where using that memory leads
> > * to crashes.
> > *
> > - * Only EFI_CONVENTIONAL_MEMORY is guaranteed to be free.
> > + * Only EFI_CONVENTIONAL_MEMORY and EFI_UNACCEPTED_MEMORY (if
> > + * supported) are guaranteed to be free.
> > */
> > - if (md->type != EFI_CONVENTIONAL_MEMORY)
> > +
> > + switch (md->type) {
> > + case EFI_CONVENTIONAL_MEMORY:
> > + break;
> > + case EFI_UNACCEPTED_MEMORY:
> > + if (IS_ENABLED(CONFIG_UNACCEPTED_MEMORY))
> > + break;
> > continue;
> > + default:
> > + continue;
> > + }
> >
> > if (efi_soft_reserve_enabled() &&
> > (md->attribute & EFI_MEMORY_SP))
> > diff --git a/arch/x86/boot/compressed/misc.c b/arch/x86/boot/compressed/misc.c
> > index fa8969fad011..c1d9d71a6615 100644
> > --- a/arch/x86/boot/compressed/misc.c
> > +++ b/arch/x86/boot/compressed/misc.c
> > @@ -18,6 +18,7 @@
> > #include "../string.h"
> > #include "../voffset.h"
> > #include <asm/bootparam_utils.h>
> > +#include <asm/unaccepted_memory.h>
> >
> > /*
> > * WARNING!!
> > @@ -43,6 +44,9 @@
> > void *memmove(void *dest, const void *src, size_t n);
> > #endif
> >
> > +#undef __pa
> > +#define __pa(x) ((unsigned long)(x))
>
> Those #undef's always worry me. Why is this one needed?

arch/x86/boot/compressed/misc.c:47:9: warning: '__pa' macro redefined [-Wmacro-redefined]
#define __pa(x) ((unsigned long)(x))
^
arch/x86/include/asm/page.h:47:9: note: previous definition is here
#define __pa(x) __phys_addr((unsigned long)(x))

Note that sev.c does the same. At least we are consistent :)

> > /*
> > * This is set up by the setup-routine at boot-time
> > */
> > @@ -451,6 +455,13 @@ asmlinkage __visible void *extract_kernel(void *rmode, memptr heap,
> > #endif
> >
> > debug_putstr("\nDecompressing Linux... ");
> > +
> > + if (IS_ENABLED(CONFIG_UNACCEPTED_MEMORY) &&
> > + boot_params->unaccepted_memory) {
> > + debug_putstr("Accepting memory... ");
> > + accept_memory(__pa(output), __pa(output) + needed_size);
> > + }
> > +
> > __decompress(input_data, input_len, NULL, NULL, output, output_len,
> > NULL, error);
> > parse_elf(output);
> > diff --git a/arch/x86/boot/compressed/unaccepted_memory.c b/arch/x86/boot/compressed/unaccepted_memory.c
> > index d363acf59c08..3ebab63789bb 100644
> > --- a/arch/x86/boot/compressed/unaccepted_memory.c
> > +++ b/arch/x86/boot/compressed/unaccepted_memory.c
> > @@ -51,3 +51,17 @@ void mark_unaccepted(struct boot_params *params, u64 start, u64 end)
> > bitmap_set((unsigned long *)params->unaccepted_memory,
> > start / PMD_SIZE, (end - start) / PMD_SIZE);
> > }
> > +
> > +void accept_memory(phys_addr_t start, phys_addr_t end)
> > +{
> > + unsigned long *unaccepted_memory;
> > + unsigned int rs, re;
> > +
> > + unaccepted_memory = (unsigned long *)boot_params->unaccepted_memory;
> > + rs = start / PMD_SIZE;
>
> OK, so start is a physical address, PMD_SIZE is 2^21, and 'rs' is an
> unsigned int. That means 'rs' can, at most, represent a physical
> address at 2^(21+32), or 2^53. That's cutting it a *bit* close, don't
> you think?
>
> Could we please just give 'rs' and 're' real names and make them
> 'unsigned long's, please? It will surely save at least one other person
> from doing math. The find_next_bit() functions seem to take ulongs anyway.

Okay. 'range_start' and 'range_end' are good enough names?

>
> > + for_each_set_bitrange_from(rs, re, unaccepted_memory,
> > + DIV_ROUND_UP(end, PMD_SIZE)) {
> > + __accept_memory(rs * PMD_SIZE, re * PMD_SIZE);
> > + bitmap_clear(unaccepted_memory, rs, re - rs);
> > + }
> > +}
>
> Could we please introduce some intermediate variable? For instance:
>
> unsigned long bitmap_size = DIV_ROUND_UP(end, PMD_SIZE);
>
> That will make this all a lot easier to read.

Okay.

>
> > diff --git a/arch/x86/include/asm/unaccepted_memory.h b/arch/x86/include/asm/unaccepted_memory.h
> > index cbc24040b853..f1f835d3cd78 100644
> > --- a/arch/x86/include/asm/unaccepted_memory.h
> > +++ b/arch/x86/include/asm/unaccepted_memory.h
> > @@ -9,4 +9,6 @@ struct boot_params;
> >
> > void mark_unaccepted(struct boot_params *params, u64 start, u64 num);
> >
> > +void accept_memory(phys_addr_t start, phys_addr_t end);
> > +
> > #endif
>

--
Kirill A. Shutemov