Re: [PATCH v2 2/2] kallsyms: handle special absolute symbols
From: Rusty Russell
Date: Wed Mar 12 2014 - 20:33:39 EST
Kees Cook <keescook@xxxxxxxxxxxx> writes:
> On Fri, Mar 7, 2014 at 5:00 PM, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>> This forces the entire per_cpu range to be reported as absolute
>> without losing their linker symbol types, when the per_cpu area is
>> 0-based. Without this, the variables are incorrectly shown as relocated
>> under kASLR on x86_64.
>>
>> Several kallsyms output in different boot states for comparison of
>> various symbols:
>>
>> $ egrep ' (gdt_|_(stext|_per_cpu_))' /root/kallsyms.nokaslr
>> 0000000000000000 D __per_cpu_start
>> 0000000000004000 D gdt_page
>> 0000000000014280 D __per_cpu_end
>> ffffffff810001c8 T _stext
>> ffffffff81ee53c0 D __per_cpu_offset
>> $ egrep ' (gdt_|_(stext|_per_cpu_))' /root/kallsyms.kaslr1
>> 000000001f200000 D __per_cpu_start
>> 000000001f204000 D gdt_page
>> 000000001f214280 D __per_cpu_end
>> ffffffffa02001c8 T _stext
>> ffffffffa10e53c0 D __per_cpu_offset
>> $ egrep ' (gdt_|_(stext|_per_cpu_))' /root/kallsyms.kaslr2
>> 000000000d400000 D __per_cpu_start
>> 000000000d404000 D gdt_page
>> 000000000d414280 D __per_cpu_end
>> ffffffff8e4001c8 T _stext
>> ffffffff8f2e53c0 D __per_cpu_offset
>> $ egrep ' (gdt_|_(stext|_per_cpu_))' /root/kallsyms.kaslr-fixed
>> 0000000000000000 D __per_cpu_start
>> 0000000000004000 D gdt_page
>> 0000000000014280 D __per_cpu_end
>> ffffffffadc001c8 T _stext
>> ffffffffaeae53c0 D __per_cpu_offset
>>
>> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
>> ---
>> v2:
>> - only force absolute when per_cpu starts at 0.
>> ---
>> scripts/kallsyms.c | 20 +++++++++++++++++++-
>> 1 file changed, 19 insertions(+), 1 deletion(-)
>>
>> diff --git a/scripts/kallsyms.c b/scripts/kallsyms.c
>> index 08f30ac5b07d..d3f93b8eb277 100644
>> --- a/scripts/kallsyms.c
>> +++ b/scripts/kallsyms.c
>> @@ -34,6 +34,7 @@ struct sym_entry {
>> unsigned int len;
>> unsigned int start_pos;
>> unsigned char *sym;
>> + int force_absolute;
>> };
>>
>> struct addr_range {
>> @@ -51,6 +52,14 @@ static struct addr_range text_ranges[] = {
>> #define text_range_text (&text_ranges[0])
>> #define text_range_inittext (&text_ranges[1])
>>
>> +/*
>> + * Variables in these ranges, when the start is 0 based, will be forced to
>> + * be handled as absolute addresses.
>> + */
>> +static struct addr_range abs_ranges[] = {
>> + { "__per_cpu_start", "__per_cpu_end", -1ULL, 0 },
>> +};
>> +
>> static struct sym_entry *table;
>> static unsigned int table_size, table_cnt;
>> static int all_symbols = 0;
>> @@ -165,6 +174,10 @@ static int read_symbol(FILE *in, struct sym_entry *s)
>> }
>> strcpy((char *)s->sym + 1, str);
>> s->sym[0] = stype;
>> + s->force_absolute = 0;
>> +
>> + /* Check if we've found special absolute symbol range. */
>> + check_symbol_range(sym, s->addr, abs_ranges, ARRAY_SIZE(abs_ranges));
>>
>> return 0;
>> }
>> @@ -211,6 +224,11 @@ static int symbol_valid(struct sym_entry *s)
>> if (s->addr < kernel_start_addr)
>> return 0;
>>
>> + /* Force zero-based range special symbols into being absolute. */
>> + i = symbol_in_range(s, abs_ranges, ARRAY_SIZE(abs_ranges));
>> + if (i >= 0 && abs_ranges[i].start == 0)
>> + s->force_absolute = 1;
>
> Rusty, is this 0-detection workable for you? If so, should you or akpm
> carry this series for 3.15?
Damn, sorry, I wrote this patch and seems like I didn't actually send it
out. No wonder you didn't respond :)
This applies on top of your first cleanup patch:
kallsyms: fix percpu vars on x86-64 with relocation.
x86-64 has a problem: per-cpu variables are actually represented by
their absolute offsets within the per-cpu area, but the symbols are
not emitted as absolute. Thus kallsyms naively creates them as offsets
from _text, meaning their values change if the kernel is relocated
(especially noticeable with CONFIG_RANDOMIZE_BASE):
$ egrep ' (gdt_|_(stext|_per_cpu_))' /root/kallsyms.nokaslr
0000000000000000 D __per_cpu_start
0000000000004000 D gdt_page
0000000000014280 D __per_cpu_end
ffffffff810001c8 T _stext
ffffffff81ee53c0 D __per_cpu_offset
$ egrep ' (gdt_|_(stext|_per_cpu_))' /root/kallsyms.kaslr1
000000001f200000 D __per_cpu_start
000000001f204000 D gdt_page
000000001f214280 D __per_cpu_end
ffffffffa02001c8 T _stext
ffffffffa10e53c0 D __per_cpu_offset
Making them absolute symbols is the Right Thing, but requires fixes to
the relocs tool. So for the moment, we add a --absolute-percpu option
which makes them absolute from a kallsyms perspective:
$ egrep ' (gdt_|_(stext|_per_cpu_))' /proc/kallsyms # no KASLR
0000000000000000 A __per_cpu_start
000000000000a000 A gdt_page
0000000000013040 A __per_cpu_end
ffffffff802001c8 T _stext
ffffffff8099b180 D __per_cpu_offset
ffffffff809a3000 D __per_cpu_load
$ egrep ' (gdt_|_(stext|_per_cpu_))' /proc/kallsyms # With KASLR
0000000000000000 A __per_cpu_start
000000000000a000 A gdt_page
0000000000013040 A __per_cpu_end
ffffffff89c001c8 T _stext
ffffffff8a39d180 D __per_cpu_offset
ffffffff8a3a5000 D __per_cpu_load
Signed-off-by: Rusty Russell <rusty@xxxxxxxxxxxxxxx>
diff --git a/scripts/kallsyms.c b/scripts/kallsyms.c
index 3c6224728960..21013e739773 100644
--- a/scripts/kallsyms.c
+++ b/scripts/kallsyms.c
@@ -51,9 +51,14 @@ static struct addr_range text_ranges[] = {
#define text_range_text (&text_ranges[0])
#define text_range_inittext (&text_ranges[1])
+static struct addr_range percpu_range = {
+ "__per_cpu_start", "__per_cpu_end", -1ULL, 0
+};
+
static struct sym_entry *table;
static unsigned int table_size, table_cnt;
static int all_symbols = 0;
+static int absolute_percpu = 0;
static char symbol_prefix_char = '\0';
static unsigned long long kernel_start_addr = 0;
@@ -166,6 +171,9 @@ static int read_symbol(FILE *in, struct sym_entry *s)
strcpy((char *)s->sym + 1, str);
s->sym[0] = stype;
+ /* Record if we've found __per_cpu_start/end. */
+ check_symbol_range(sym, s->addr, &percpu_range, 1);
+
return 0;
}
@@ -656,6 +664,15 @@ static void sort_symbols(void)
qsort(table, table_cnt, sizeof(struct sym_entry), compare_symbols);
}
+static void make_percpus_absolute(void)
+{
+ unsigned int i;
+
+ for (i = 0; i < table_cnt; i++)
+ if (symbol_in_range(&table[i], &percpu_range, 1))
+ table[i].sym[0] = 'A';
+}
+
int main(int argc, char **argv)
{
if (argc >= 2) {
@@ -663,6 +683,8 @@ int main(int argc, char **argv)
for (i = 1; i < argc; i++) {
if(strcmp(argv[i], "--all-symbols") == 0)
all_symbols = 1;
+ else if (strcmp(argv[i], "--absolute-percpu") == 0)
+ absolute_percpu = 1;
else if (strncmp(argv[i], "--symbol-prefix=", 16) == 0) {
char *p = &argv[i][16];
/* skip quote */
@@ -679,6 +701,8 @@ int main(int argc, char **argv)
usage();
read_map(stdin);
+ if (absolute_percpu)
+ make_percpus_absolute();
sort_symbols();
optimize_token_table();
write_src();
diff --git a/scripts/link-vmlinux.sh b/scripts/link-vmlinux.sh
index 2dcb37736d84..86a4fe75f453 100644
--- a/scripts/link-vmlinux.sh
+++ b/scripts/link-vmlinux.sh
@@ -86,6 +86,10 @@ kallsyms()
kallsymopt="${kallsymopt} --page-offset=$CONFIG_PAGE_OFFSET"
fi
+ if [ -n "${CONFIG_X86_64}" ]; then
+ kallsymopt="${kallsymopt} --absolute-percpu"
+ fi
+
local aflags="${KBUILD_AFLAGS} ${KBUILD_AFLAGS_KERNEL} \
${NOSTDINC_FLAGS} ${LINUXINCLUDE} ${KBUILD_CPPFLAGS}"
--
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