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Date: Fri, 20 May 2022 18:48:41 +0000 From: Sean Christopherson <seanjc@...gle.com> To: Kees Cook <keescook@...omium.org> Cc: Paolo Bonzini <pbonzini@...hat.com>, Vitaly Kuznetsov <vkuznets@...hat.com>, Wanpeng Li <wanpengli@...cent.com>, Jim Mattson <jmattson@...gle.com>, x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>, kvm@...r.kernel.org, Joerg Roedel <joro@...tes.org>, linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org Subject: Re: [PATCH] KVM: x86/emulator: Bounds check reg nr against reg array size On Fri, May 20, 2022, Kees Cook wrote: > GCC 12 sees that it might be possible for "nr" to be outside the _regs > array. Add explicit bounds checking. > > In function 'reg_read', > inlined from 'reg_rmw' at ../arch/x86/kvm/emulate.c:266:2: > ../arch/x86/kvm/emulate.c:254:27: warning: array subscript 32 is above array bounds of 'long unsigned int[17]' [-Warray-bounds] > 254 | return ctxt->_regs[nr]; > | ~~~~~~~~~~~^~~~ > In file included from ../arch/x86/kvm/emulate.c:23: > ../arch/x86/kvm/kvm_emulate.h: In function 'reg_rmw': > ../arch/x86/kvm/kvm_emulate.h:366:23: note: while referencing '_regs' > 366 | unsigned long _regs[NR_VCPU_REGS]; > | ^~~~~ > > Cc: Paolo Bonzini <pbonzini@...hat.com> > Cc: Sean Christopherson <seanjc@...gle.com> > Cc: Vitaly Kuznetsov <vkuznets@...hat.com> > Cc: Wanpeng Li <wanpengli@...cent.com> > Cc: Jim Mattson <jmattson@...gle.com> > Cc: x86@...nel.org > Cc: "H. Peter Anvin" <hpa@...or.com> > Cc: kvm@...r.kernel.org > Signed-off-by: Kees Cook <keescook@...omium.org> > --- > arch/x86/kvm/emulate.c | 4 ++++ > 1 file changed, 4 insertions(+) > > diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c > index 89b11e7dca8a..fbcbc012a3ae 100644 > --- a/arch/x86/kvm/emulate.c > +++ b/arch/x86/kvm/emulate.c > @@ -247,6 +247,8 @@ enum x86_transfer_type { > > static ulong reg_read(struct x86_emulate_ctxt *ctxt, unsigned nr) > { > + if (WARN_ON(nr >= ARRAY_SIZE(ctxt->_regs))) > + return 0; > if (!(ctxt->regs_valid & (1 << nr))) { > ctxt->regs_valid |= 1 << nr; > ctxt->_regs[nr] = ctxt->ops->read_gpr(ctxt, nr); > @@ -256,6 +258,8 @@ static ulong reg_read(struct x86_emulate_ctxt *ctxt, unsigned nr) > > static ulong *reg_write(struct x86_emulate_ctxt *ctxt, unsigned nr) > { > + if (WARN_ON(nr >= ARRAY_SIZE(ctxt->_regs))) > + return 0; This is wrong, reg_write() confusingly returns a pointer the register to be written, it doesn't actually do the write. So if we want to guard against array overflow, it would be better to cap @nr and continue on, i.e. assume some higher bit was spuriously set. The other oddity here is that VCPU_REGS_RIP should never be read, the RIP relative code reads _eip directly. I.e. _regs[] should really be VCPU_REGS_R15+1. And adding a #define for that would clean up this bit of code in writeback_registers() that hardcodes 16 (rax - r15) GPRs: for_each_set_bit(reg, (ulong *)&ctxt->regs_dirty, 16) ctxt->ops->write_gpr(ctxt, reg, ctxt->_regs[reg]); Lastly, casting regs_dirty to an unsigned long pointer is all kinds of gross, e.g. if it were moved to the end of struct x86_emulate_ctxt then the above could trigger an out-of-bounds read. I'll whip up a small series to clean this code up and add WARNs similar to above.
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