Re: [PATCH] linux-user/elfload: munmap proper address in pgd_find_hole_fallback

[Laurent Vivier <laurent@vivier.eu>]

Le 31/01/2021 à 07:18, Vincent Fazio a écrit :
> From: Vincent Fazio <vfazio@gmail.com>
> 
> Previously, if the build host's libc did not define MAP_FIXED_NOREPLACE
> or if the running kernel didn't support that flag, it was possible for
> pgd_find_hole_fallback to munmap an incorrect address which could lead to
> SIGSEGV if the range happened to overlap with the mapped address of the
> QEMU binary.
> 
>   mmap(0x1000, 22261224, PROT_NONE, MAP_PRIVATE|MAP_ANONYMOUS|MAP_NORESERVE, -1, 0) = 0x7f889d331000
>   munmap(0x1000, 22261224)                = 0
>   --- SIGSEGV {si_signo=SIGSEGV, si_code=SEGV_MAPERR, si_addr=0x84b817} ---
>   ++ killed by SIGSEGV +++
> 
> Now, always munmap the address returned by mmap.
> 
> Fixes: 2667e069e7b5 ("linux-user: don't use MAP_FIXED in pgd_find_hole_fallback")
> Signed-off-by: Vincent Fazio <vfazio@gmail.com>
> ---
>  linux-user/elfload.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/linux-user/elfload.c b/linux-user/elfload.c
> index a64050713f..5f5f23d2e5 100644
> --- a/linux-user/elfload.c
> +++ b/linux-user/elfload.c
> @@ -2216,7 +2216,7 @@ static uintptr_t pgd_find_hole_fallback(uintptr_t guest_size, uintptr_t brk,
>              void * mmap_start = mmap((void *) align_start, guest_size,
>                                       PROT_NONE, flags, -1, 0);
>              if (mmap_start != MAP_FAILED) {
> -                munmap((void *) align_start, guest_size);
> +                munmap(mmap_start, guest_size);
>                  if (MAP_FIXED_NOREPLACE != 0 ||
>                      mmap_start == (void *) align_start) {
>                      return (uintptr_t) mmap_start + offset;
> 

Applied to my linux-user-for-6.0 branch.

Thanks,
Laurent