Re: [RFC][PATCH] mm: Tighten x86 /dev/mem with zeroing

[Tommi Rantala]

On 06.04.2017 03:00, Kees Cook wrote:
> This changes the x86 exception for the low 1MB by reading back zeros for
> RAM areas instead of blindly allowing them. (It may be possible for heap
> to end up getting allocated in low 1MB RAM, and then read out, possibly
> tripping hardened usercopy.)
>
> Unfinished: this still needs mmap support.
>
> Reported-by: Tommi Rantala <tommi.t.rantala@xxxxxxxxx>
> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> ---
> Tommi, can you check and see if this fixes what you're seeing? I want to
> make sure this actually works first. (x86info uses seek/read not mmap.)

Hi, I can confirm that it works (after adding CONFIG_STRICT_DEVMEM), no 
more kernel bugs when running x86info.


open("/dev/mem", O_RDONLY)              = 3
lseek(3, 1038, SEEK_SET)                = 1038
read(3, "\300\235", 2)                  = 2
lseek(3, 646144, SEEK_SET)              = 646144
read(3, 
"\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 
1024) = 1024
lseek(3, 1043, SEEK_SET)                = 1043
read(3, "w\2", 2)                       = 2
lseek(3, 645120, SEEK_SET)              = 645120
read(3, 
"\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 
1024) = 1024
lseek(3, 654336, SEEK_SET)              = 654336
read(3, 
"\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 
1024) = 1024
lseek(3, 983040, SEEK_SET)              = 983040
read(3, 
"IFE$\245S\0\0\1\0\0\0\0\360y\0\0\360\220\260\30\237{=\23\10\17\0000\276\17\0"..., 
65536) = 65536
lseek(3, 917504, SEEK_SET)              = 917504
read(3, 
"\377\377\377\377\377\377\377\377\377\377\377\377\377\377\377\377\377\377\377\377\377\377\377\377\377\377\377\377\377\377\377\377"..., 
65536) = 65536
lseek(3, 524288, SEEK_SET)              = 524288
read(3, 
"\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 
65536) = 65536
lseek(3, 589824, SEEK_SET)              = 589824
read(3, 
"\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 
65536) = 65536


dd works too:

# LANG=C dd if=/dev/mem of=/dev/null bs=4096 count=256
256+0 records in
256+0 records out
1048576 bytes (1.0 MB, 1.0 MiB) copied, 0.0874073 s, 12.0 MB/s


> ---
>
>  arch/x86/mm/init.c | 41 +++++++++++++++++++--------
>  drivers/char/mem.c | 82 ++++++++++++++++++++++++++++++++++--------------------
>  2 files changed, 82 insertions(+), 41 deletions(-)