116 lines
4.1 KiB
Diff
116 lines
4.1 KiB
Diff
From: Kees Cook <keescook@chromium.org>
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Date: Thu, 6 Dec 2012 17:00:21 +1100
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Subject: [1/2] exec: do not leave bprm->interp on stack
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commit 1e1b8374592f5fb347625e84d8a5f2f40d858a24 upstream.
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If a series of scripts are executed, each triggering module loading via
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unprintable bytes in the script header, kernel stack contents can leak
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into the command line.
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Normally execution of binfmt_script and binfmt_misc happens recursively.
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However, when modules are enabled, and unprintable bytes exist in the
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bprm->buf, execution will restart after attempting to load matching binfmt
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modules. Unfortunately, the logic in binfmt_script and binfmt_misc does
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not expect to get restarted. They leave bprm->interp pointing to their
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local stack. This means on restart bprm->interp is left pointing into
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unused stack memory which can then be copied into the userspace argv
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areas.
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After additional study, it seems that both recursion and restart remains
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the desirable way to handle exec with scripts, misc, and modules. As
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such, we need to protect the changes to interp.
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This changes the logic to require allocation for any changes to the
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bprm->interp. To avoid adding a new kmalloc to every exec, the default
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value is left as-is. Only when passing through binfmt_script or
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binfmt_misc does an allocation take place.
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For a proof of concept, see DoTest.sh from:
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http://www.halfdog.net/Security/2012/LinuxKernelBinfmtScriptStackDataDisclosure/
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Signed-off-by: Kees Cook <keescook@chromium.org>
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Cc: halfdog <me@halfdog.net>
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Cc: P J P <ppandit@redhat.com>
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Cc: Alexander Viro <viro@zeniv.linux.org.uk>
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Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
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---
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fs/binfmt_misc.c | 5 ++++-
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fs/binfmt_script.c | 4 +++-
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fs/exec.c | 15 +++++++++++++++
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include/linux/binfmts.h | 1 +
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4 files changed, 23 insertions(+), 2 deletions(-)
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diff --git a/fs/binfmt_misc.c b/fs/binfmt_misc.c
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index b0b70fb..b0c1755 100644
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--- a/fs/binfmt_misc.c
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+++ b/fs/binfmt_misc.c
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@@ -176,7 +176,10 @@ static int load_misc_binary(struct linux_binprm *bprm)
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goto _error;
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bprm->argc ++;
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- bprm->interp = iname; /* for binfmt_script */
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+ /* Update interp in case binfmt_script needs it. */
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+ retval = bprm_change_interp(iname, bprm);
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+ if (retval < 0)
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+ goto _error;
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interp_file = open_exec (iname);
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retval = PTR_ERR (interp_file);
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diff --git a/fs/binfmt_script.c b/fs/binfmt_script.c
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index 8c95499..4834f2c 100644
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--- a/fs/binfmt_script.c
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+++ b/fs/binfmt_script.c
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@@ -82,7 +82,9 @@ static int load_script(struct linux_binprm *bprm)
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retval = copy_strings_kernel(1, &i_name, bprm);
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if (retval) return retval;
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bprm->argc++;
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- bprm->interp = interp;
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+ retval = bprm_change_interp(interp, bprm);
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+ if (retval < 0)
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+ return retval;
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/*
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* OK, now restart the process with the interpreter's dentry.
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diff --git a/fs/exec.c b/fs/exec.c
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index b71b08c..bf50973 100644
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--- a/fs/exec.c
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+++ b/fs/exec.c
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@@ -1175,9 +1175,24 @@ void free_bprm(struct linux_binprm *bprm)
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mutex_unlock(¤t->signal->cred_guard_mutex);
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abort_creds(bprm->cred);
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}
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+ /* If a binfmt changed the interp, free it. */
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+ if (bprm->interp != bprm->filename)
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+ kfree(bprm->interp);
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kfree(bprm);
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}
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+int bprm_change_interp(char *interp, struct linux_binprm *bprm)
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+{
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+ /* If a binfmt changed the interp, free it first. */
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+ if (bprm->interp != bprm->filename)
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+ kfree(bprm->interp);
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+ bprm->interp = kstrdup(interp, GFP_KERNEL);
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+ if (!bprm->interp)
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+ return -ENOMEM;
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+ return 0;
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+}
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+EXPORT_SYMBOL(bprm_change_interp);
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+
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/*
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* install the new credentials for this executable
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*/
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diff --git a/include/linux/binfmts.h b/include/linux/binfmts.h
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index 2630c9b..7f0e297 100644
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--- a/include/linux/binfmts.h
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+++ b/include/linux/binfmts.h
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@@ -114,6 +114,7 @@ extern int setup_arg_pages(struct linux_binprm * bprm,
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unsigned long stack_top,
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int executable_stack);
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extern int bprm_mm_init(struct linux_binprm *bprm);
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+extern int bprm_change_interp(char *interp, struct linux_binprm *bprm);
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extern int copy_strings_kernel(int argc, const char *const *argv,
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struct linux_binprm *bprm);
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extern int prepare_bprm_creds(struct linux_binprm *bprm);
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