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livepatch,x86: Clear relocation targets on a module removal

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Josh reported a bug:

  When the object to be patched is a module, and that module is
  rmmod'ed and reloaded, it fails to load with:

  module: x86/modules: Skipping invalid relocation target, existing value is nonzero for type 2, loc 00000000ba0302e9, val ffffffffa03e293c
  livepatch: failed to initialize patch 'livepatch_nfsd' for module 'nfsd' (-8)
  livepatch: patch 'livepatch_nfsd' failed for module 'nfsd', refusing to load module 'nfsd'

  The livepatch module has a relocation which references a symbol
  in the _previous_ loading of nfsd. When apply_relocate_add()
  tries to replace the old relocation with a new one, it sees that
  the previous one is nonzero and it errors out.

He also proposed three different solutions. We could remove the error
check in apply_relocate_add() introduced by commit eda9cec4c9
("x86/module: Detect and skip invalid relocations"). However the check
is useful for detecting corrupted modules.

We could also deny the patched modules to be removed. If it proved to be
a major drawback for users, we could still implement a different
approach. The solution would also complicate the existing code a lot.

We thus decided to reverse the relocation patching (clear all relocation
targets on x86_64). The solution is not
universal and is too much arch-specific, but it may prove to be simpler
in the end.

Reported-by: Josh Poimboeuf <jpoimboe@redhat.com>
Originally-by: Miroslav Benes <mbenes@suse.cz>
Signed-off-by: Song Liu <song@kernel.org>
Acked-by: Miroslav Benes <mbenes@suse.cz>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Acked-by: Josh Poimboeuf <jpoimboe@kernel.org>
Reviewed-by: Joe Lawrence <joe.lawrence@redhat.com>
Tested-by: Joe Lawrence <joe.lawrence@redhat.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20230125185401.279042-2-song@kernel.org
This commit is contained in:
Song Liu
2023-01-25 10:54:01 -08:00
committed by Petr Mladek
parent bbb93362a4
commit 0c05e7bd2d
3 changed files with 126 additions and 46 deletions
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17
include/linux/moduleloader.h
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@@ -72,6 +72,23 @@ int apply_relocate_add(Elf_Shdr *sechdrs,
unsigned int symindex,
unsigned int relsec,
struct module *mod);
#ifdef CONFIG_LIVEPATCH
/*
* Some architectures (namely x86_64 and ppc64) perform sanity checks when
* applying relocations. If a patched module gets unloaded and then later
* reloaded (and re-patched), klp re-applies relocations to the replacement
* function(s). Any leftover relocations from the previous loading of the
* patched module might trigger the sanity checks.
*
* To prevent that, when unloading a patched module, clear out any relocations
* that might trigger arch-specific sanity checks on a future module reload.
*/
void clear_relocate_add(Elf_Shdr *sechdrs,
const char *strtab,
unsigned int symindex,
unsigned int relsec,
struct module *me);
#endif
#else
static inline int apply_relocate_add(Elf_Shdr *sechdrs,
const char *strtab,