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SubjectRe: [PATCH v3 3/9] livepatch: Add klp-convert tool
On Wed, Apr 10, 2019 at 11:50:52AM -0400, Joe Lawrence wrote:
>
> [ ... snip ... ]
>
> +static bool convert_rela(struct section *oldsec, struct rela *r,
> + struct sympos *sp, struct elf *klp_elf)
> +{
> + struct section *sec;
> + struct rela *r1, *r2;
> +
> + sec = get_or_create_klp_rela_section(oldsec, sp, klp_elf);
> + if (!sec) {
> + WARN("Can't create or access klp.rela section (%s.%s)\n",
> + sp->object_name, sp->symbol_name);
> + return false;
> + }
> +
> + if (!convert_klp_symbol(r->sym, sp)) {
> + WARN("Unable to convert symbol name (%s.%s)\n", sec->name,
> + r->sym->name);
> + return false;
> + }
> +
> + /* Move the converted rela to klp rela section */
> + list_for_each_entry_safe(r1, r2, &oldsec->relas, list) {
> + if (r1->sym->name == r->sym->name) {
> + list_del(&r1->list);
> + list_add(&r1->list, &sec->relas);
> + }
> + }
> + return true;
> +}

This one took a while to find and debug, but I believe that
convert_rela()'s list removal is not as safe as it thinks it is.

Start with its calling context from main() below:

> + list_for_each_entry_safe(sec, aux, &klp_elf->sections, list) {
> + if (!is_rela_section(sec))
> + continue;
> +
> + list_for_each_entry_safe(rela, tmprela, &sec->relas, list) {
> + if (!must_convert(rela->sym))
> + continue;
> +
> + if (!find_missing_position(rela->sym, &sp)) {
> + WARN("Unable to find missing symbol: %s",
> + rela->sym->name);
> + return -1;
> + }
> + if (!convert_rela(sec, rela, &sp, klp_elf)) {
> + WARN("Unable to convert relocation: %s",
> + rela->sym->name);
> + return -1;
> + }
> + }
> + }

AFAIK the *_safe list traversals, they cache the ->next value at the
beginning of each iteration, so that one could blow the current element
in position away. The cached ->next value is then assigned when moving
to the next iteration.

But notice how convert_rela() looks through the entire list of
relocations, moving each rela with a matching symbol?

Consider a slight tweak to samples/livepatch-annotated.c:


static int livepatch_cmdline_proc_show(struct seq_file *m, void *v)
{
+ if (saved_command_line)
+ saved_command_line[0] = '\0';
+
seq_printf(m, "%s livepatch=1\n", saved_command_line);
return 0;
}


On my system, this generates relocations like this:

% eu-readelf --relocs samples/livepatch/livepatch-annotated-sample.o
Relocation section [ 2] '.rela.text' for section [ 1] '.text' at offset 0x98 contains 9 entries:
Offset Type Value Addend Name
0x0000000000000001 X86_64_PC32 000000000000000000 -4 __fentry__
0x0000000000000008 X86_64_PC32 000000000000000000 -4 saved_command_line
0x0000000000000017 X86_64_PC32 000000000000000000 -4 saved_command_line
0x000000000000001e X86_64_32S 000000000000000000 +0 .rodata.str1.1
0x0000000000000023 X86_64_PC32 000000000000000000 -4 seq_printf
0x0000000000000031 X86_64_PC32 000000000000000000 -4 __fentry__
0x0000000000000038 X86_64_32S 000000000000000000 +0 .data
0x0000000000000051 X86_64_PC32 000000000000000000 -4 __fentry__
0x000000000000003d X86_64_PC32 000000000000000000 -4 klp_enable_patch

We now have back-to-back rela's with sym->name = "saved_command_line".
When the first is converted, convert_rela() will move both of them to
the klp rela section. The linked list values may be consistent, but the
cached ->next value will be bogus and the in-flight-traversal will run
off the rails.


I think we can work around it with a combination of 1) only moving a
single rela symbol at a time in convert_rela and 2) processing the
second (third, etc.) a little bit more so that they are moved
individually.

I hacked this together and it works against the livepatch-annotate.c
test above so far...

-->8-- -->8-- -->8-- -->8-- -->8-- -->8-- -->8-- -->8-- -->8-- -->8--

diff --git a/scripts/livepatch/klp-convert.c b/scripts/livepatch/klp-convert.c
index 82c27d219372..126395f1c0cd 100644
--- a/scripts/livepatch/klp-convert.c
+++ b/scripts/livepatch/klp-convert.c
@@ -517,6 +517,7 @@ static bool convert_rela(struct section *oldsec, struct rela *r,
if (r1->sym->name == r->sym->name) {
list_del(&r1->list);
list_add(&r1->list, &sec->relas);
+ break;
}
}
return true;
@@ -549,8 +550,8 @@ static bool is_converted(char *sname)
}

/*
- * Checks if symbol must be converted (conditions):
- * not resolved, not already converted or isn't an exported symbol
+ * Checks if symbol must be or was already converted (conditions):
+ * not resolved or isn't an exported symbol
*/
static bool must_convert(struct symbol *sym)
{
@@ -566,7 +567,7 @@ static bool must_convert(struct symbol *sym)
if (strcmp(sym->name, ".TOC.") == 0)
return false;

- return (!(is_converted(sym->name) || is_exported(sym->name)));
+ return (!is_exported(sym->name));
}

/* Checks if a section is a klp rela section */
@@ -640,7 +641,8 @@ int main(int argc, const char **argv)
if (!must_convert(rela->sym))
continue;

- if (!find_missing_position(rela->sym, &sp)) {
+ if (!is_converted(rela->sym->name) &&
+ !find_missing_position(rela->sym, &sp)) {
WARN("Unable to find missing symbol: %s",
rela->sym->name);
return -1;
--
2.20.1
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