linux-kernel - Re: [PATCH] kexec: Support purgatories with .text.hot sections

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Message-ID: <20230322201303.GA2979303@google.com>
Date:   Wed, 22 Mar 2023 14:13:03 -0600
From:   Ross Zwisler <zwisler@...gle.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Ricardo Ribalda <ribalda@...omium.org>,
        Eric Biederman <ebiederm@...ssion.com>,
        Philipp Rudo <prudo@...ux.vnet.ibm.com>,
        linux-kernel@...r.kernel.org, kexec@...ts.infradead.org
Subject: Re: [PATCH] kexec: Support purgatories with .text.hot sections

On Wed, Mar 22, 2023 at 10:34:46AM -0400, Steven Rostedt wrote:
> On Tue, Mar 21, 2023 at 12:49:08PM +0100, Ricardo Ribalda wrote:
> > Clang16 links the purgatory text in two sections:
> > 
> >   [ 1] .text             PROGBITS         0000000000000000  00000040
> >        00000000000011a1  0000000000000000  AX       0     0     16
> >   [ 2] .rela.text        RELA             0000000000000000  00003498
> >        0000000000000648  0000000000000018   I      24     1     8
> >   ...
> >   [17] .text.hot.        PROGBITS         0000000000000000  00003220
> >        000000000000020b  0000000000000000  AX       0     0     1
> >   [18] .rela.text.hot.   RELA             0000000000000000  00004428
> >        0000000000000078  0000000000000018   I      24    17     8
> > 
> > And both of them have their range [sh_addr ... sh_addr+sh_size] on the
> > area pointed by `e_entry`.
> > 
> > This causes that image->start is calculated twice, once for .text and
> > another time for .text.hot. The second calculation leaves image->start
> > in a random location.
> > 
> > Because of this, the system crashes inmediatly after:
> > 
> > kexec_core: Starting new kernel
> > 
> > Signed-off-by: Ricardo Ribalda <ribalda@...omium.org>
> > To: Eric Biederman <ebiederm@...ssion.com>
> > Cc: Philipp Rudo <prudo@...ux.vnet.ibm.com>
> > Cc: kexec@...ts.infradead.org
> > Cc: linux-kernel@...r.kernel.org
> > ---
> >  kernel/kexec_file.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> > 
> > diff --git a/kernel/kexec_file.c b/kernel/kexec_file.c
> > index f1a0e4e3fb5c..b1a25d97d5e2 100644
> > --- a/kernel/kexec_file.c
> > +++ b/kernel/kexec_file.c
> > @@ -904,7 +904,8 @@ static int kexec_purgatory_setup_sechdrs(struct purgatory_info *pi,
> >  		if (sechdrs[i].sh_flags & SHF_EXECINSTR &&
> >  		    pi->ehdr->e_entry >= sechdrs[i].sh_addr &&
> >  		    pi->ehdr->e_entry < (sechdrs[i].sh_addr
> > -					 + sechdrs[i].sh_size)) {
> > +					 + sechdrs[i].sh_size) &&
> > +		    kbuf->image->start != pi->ehdr->e_shnum) {

I think we should also be comparing against the initial value (set ~20 lines
above) of pi->ehdr->e_entry, not pi->ehdr->e_shnum.

This patch works correctly for me:

diff --git a/kernel/kexec_file.c b/kernel/kexec_file.c
index f7a4fd4d243f4..967826a42cdd7 100644
--- a/kernel/kexec_file.c
+++ b/kernel/kexec_file.c
@@ -913,7 +913,8 @@ static int kexec_purgatory_setup_sechdrs(struct purgatory_info *pi,
                if (sechdrs[i].sh_flags & SHF_EXECINSTR &&
                    pi->ehdr->e_entry >= sechdrs[i].sh_addr &&
                    pi->ehdr->e_entry < (sechdrs[i].sh_addr
-                                        + sechdrs[i].sh_size)) {
+                                        + sechdrs[i].sh_size) &&
+                   kbuf->image->start == pi->ehdr->e_entry) {
                        kbuf->image->start -= sechdrs[i].sh_addr;
                        kbuf->image->start += kbuf->mem + offset;
                }

Great find.  With those 2 quick changes, you can add:

Reviewed-by: Ross Zwisler <zwisler@...gle.com>

> Shouldn't this be: kbuf->image->start == pi->ehdr->e_shnum) {
> 
> ?
> 
> As you want to only do this update when it's not equal to the initial value.
> If this did work, then you may want to make sure that was the initial value.
> 
> Also, please add a comment about why you are doing this check.
> 
> Thanks!
> 
> -- Steve
> 
> >  			kbuf->image->start -= sechdrs[i].sh_addr;
> >  			kbuf->image->start += kbuf->mem + offset;
> >  		}
> > 
> > ---
> > base-commit: 17214b70a159c6547df9ae204a6275d983146f6b
> > change-id: 20230321-kexec_clang16-4510c23d129c
> > 
> > Best regards,
> > -- 
> > Ricardo Ribalda <ribalda@...omium.org>