linux-kernel - Re: [PATCH] x86/efi: Free allocated memory if remap fails

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Date:   Mon, 18 Jun 2018 10:20:39 -0700
From:   Sai Praneeth Prakhya <sai.praneeth.prakhya@...el.com>
To:     Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc:     linux-efi <linux-efi@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Lee Chun-Yi <jlee@...e.com>, Borislav Petkov <bp@...en8.de>,
        Tony Luck <tony.luck@...el.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Bhupesh Sharma <bhsharma@...hat.com>,
        Ricardo Neri <ricardo.neri@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ravi Shankar <ravi.v.shankar@...el.com>,
        Matt Fleming <matt@...eblueprint.co.uk>
Subject: Re: [PATCH] x86/efi: Free allocated memory if remap fails


> > It's a fact that memremap() and early_memremap() might never fail and
> > this code might never get a chance to run but to maintain good kernel
> > programming semantics, we might need this patch.
> > 
> > Signed-off-by: Sai Praneeth Prakhya <sai.praneeth.prakhya@...el.com>
> > Reviewed-by: Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
> Please don't include tags for reviews that did not happen on-list.
> 

Sure! Thanks for letting me know.

> > @@ -450,10 +451,11 @@ void __init efi_free_boot_services(void)
> > 
> >         memunmap(new);
> > 
> > -       if (efi_memmap_install(new_phys, num_entries)) {
> > +       if (efi_memmap_install(new_phys, num_entries))
> >                 pr_err("Could not install new EFI memmap\n");
> > -               return;
> > -       }
> > +
> > +free_mem:
> > +       efi_memmap_free(new_phys, num_entries);
> Doesn't this free the memory map that you just installed?
> 

That's true! It's a bug. I will fix it.

> > 
> >  }
> > 
> >  /**
> > + * efi_memmap_free - Free memory allocated by efi_memmap_alloc()
> > + * @mem: Physical address allocated by efi_memmap_alloc()
> > + * @num_entries: Number of entries in the allocated map.
> > + *
> > + * efi_memmap_alloc() allocates memory depending on whether mm_init()
> > + * has already been invoked or not. It uses either memblock or "normal"
> > + * page allocation. Use this function to free the memory allocated by
> > + * efi_memmap_alloc(). Since the allocation is done in two different
> > + * ways, similarly, we free it in two different ways.
> > + *
> > + */
> > +void __init efi_memmap_free(phys_addr_t mem, unsigned int num_entries)
> > +{
> > +       unsigned long size = num_entries * efi.memmap.desc_size;
> > +       unsigned int order = get_order(size);
> > +       phys_addr_t end = mem + size - 1;
> > +
> > +       if (slab_is_available()) {
> > +               __free_pages(pfn_to_page(PHYS_PFN(mem)), order);
> How do you know that the memory you are freeing was allocated when
> slab_is_available() was already true?
> 

efi_memmap_free() should be used *only* in conjunction
with efi_memmap_alloc()(As I explicitly didn't mention this, maybe it might
have confused you).

When allocating memory efi_memmap_alloc() does similar check
for slab_is_available() and if so, it allocates memory using alloc_pages().
So, to free pages allocated using alloc_pages(), efi_memmap_free()
uses __free_pages().

> > 
> > +               return;
> > +       }
> > +
> > +       if (memblock_free(mem, size))
> > +               pr_err("Failed to free mem from %pa to %pa\n", &mem,
> > &end);
> > +}
> > +

Regards,
Sai