Messages in this thread Patch in this message | | | Date | Sat, 1 Aug 2009 10:52:08 -0700 (PDT) | From | Linus Torvalds <> | Subject | Re: [PATCH] information leak in sigaltstack |
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On Fri, 31 Jul 2009, Linus Torvalds wrote: > > Here's the patch I used, just for posterity. I can't decide if I really > want to commit this crap.
I think I like this version better. It's not wonderfully pretty, but to some degree it actually improves on code generation, even if it generates a bigger exception table. It now also copies the stack_t back to user space the same way it copies it _from_ user space, so in that sense it's actually very consistent.
Linus --- From: Linus Torvalds <torvalds@linux-foundation.org> Date: Sat, 1 Aug 2009 10:34:56 -0700 Subject: [PATCH] do_sigaltstack: avoid copying 'stack_t' as a structure to user space
Ulrich Drepper correctly points out that there is generally padding in the structure on 64-bit hosts, and that copying the structure from kernel to user space can leak information from the kernel stack in those padding bytes.
Avoid the whole issue by just copying the three members one by one instead, which also means that the function also can avoid the need for a stack frame. This also happens to match how we copy the new structure from user space, so it all even makes sense.
[ The obvious solution of adding a memset() generates horrid code, gcc does really stupid things. ]
Reported-by: Ulrich Drepper <drepper@redhat.com> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org> --- kernel/signal.c | 15 ++++++++------- 1 files changed, 8 insertions(+), 7 deletions(-)
diff --git a/kernel/signal.c b/kernel/signal.c index ccf1cee..f268372 100644 --- a/kernel/signal.c +++ b/kernel/signal.c @@ -2454,11 +2454,9 @@ do_sigaltstack (const stack_t __user *uss, stack_t __user *uoss, unsigned long s stack_t oss; int error; - if (uoss) { - oss.ss_sp = (void __user *) current->sas_ss_sp; - oss.ss_size = current->sas_ss_size; - oss.ss_flags = sas_ss_flags(sp); - } + oss.ss_sp = (void __user *) current->sas_ss_sp; + oss.ss_size = current->sas_ss_size; + oss.ss_flags = sas_ss_flags(sp); if (uss) { void __user *ss_sp; @@ -2501,13 +2499,16 @@ do_sigaltstack (const stack_t __user *uss, stack_t __user *uoss, unsigned long s current->sas_ss_size = ss_size; } + error = 0; if (uoss) { error = -EFAULT; - if (copy_to_user(uoss, &oss, sizeof(oss))) + if (!access_ok(VERIFY_WRITE, uoss, sizeof(*uoss))) goto out; + error = __put_user(oss.ss_sp, &uoss->ss_sp) | + __put_user(oss.ss_size, &uoss->ss_size) | + __put_user(oss.ss_flags, &uoss->ss_flags); } - error = 0; out: return error; }
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